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QT114-DG

型号：

QT114-DG

描述：

QLEVEL⑩传感器IC[ QLEVEL⑩ SENSOR IC ]

品牌：

QUALCOMM[ QUALCOMM INCORPORATED ]

页数：

14 页

PDF大小：

247 K

下载PDF手册
lQ  
QT114  
QLEVEL™ SENSOR IC  
" Limit sensing of almost any fluid or powder  
" Two-tier level sensor - Hi / Low limits with one probe  
" Only one external part required - a 5¢ capacitor  
" Uses internal probes or external electrodes  
" Active high or active low outputs  
" Slosh filter averages response of moving fluids  
" LED drive capable on both outputs  
" 2.5 to 5V 20µA single supply operation  
" HeartBeat™ health indicator on both outputs  
" RoHS compliant package  
Vcc  
Out1  
Out2  
Filt  
1
2
3
4
8
7
6
5
Gnd  
Sns2  
Sns1  
Pol  
APPLICATIONS -  
! Process controls  
! Vending machines  
! Consumer appliances  
! Automotive fluids  
! Medical fluid sensing  
! Soil moisture sensing  
DESCRIPTION -  
The QT114 QLevel™ charge-transfer (“QT”) sensor IC is specifically designed to detect point level in fluids and  
powders. It will project a sense field through almost any dielectric, like glass, plastic, or ceramic, to sense level on  
the inside of a vessel, from its exterior. It has the unique capability of independently sensing two trip points when  
used with structured electrodes having two tiers.  
The QT114 does not have sensing timeouts, drift compensation, or other functions which would interfere with level  
sensing. Its threshold levels are fixed, and the amount of signal required to exceed a threshold is dependent on  
circuit gain and electrode size and loading, all of which are under the control of the designer.  
The QT114 requires only a single inexpensive capacitor in order to function. One or two LEDs can also be added to  
provide a visual sensing indication.  
Power consumption is under 20mA in most applications, allowing operation from Lithium cells for many years. In  
most cases the power supply needs only minimal regulation.  
The QT114 employs numerous signal acquisition and processing techniques pioneered by Quantum. No external  
switches, opamps, or other analog components aside from CS are required.  
A unique feature is the 'slosh filter', a detection integrator which averages detections over a rolling 15 second  
interval before activating or deactivating the OUT pins. This filter allows use of the QT114 with violently moving  
fluids, for example in a moving vehicle, that would otherwise cause the outputs to flicker between two states.  
The device also includes selectable output polarity, allowing both output lines to be made either active-high or  
active-low. It also includes the Quantum-pioneered HeartBeat™ signal, allowing a host controller to monitor the  
health of the QT114 continuously if desired. By using the charge transfer principle, the IC delivers a level of  
performance clearly superior to older technologies. It is specifically designed to replace electromechanical devices  
like float switches, thermistors, and conductance probes.  
AVAILABLE OPTIONS  
TA  
SOIC  
8-PIN DIP  
QT114-DG  
-
00C to +700C  
-400C to +850C  
-
QT114-ISG  
lq  
Copyright © 1999-2006 QRG Ltd  
QT114 R1.04/1106  
The QT114 is a digital burst mode charge-transfer (QT)  
sensor designed specifically for point level sensing; it  
includes all hardware and signal processing functions  
necessary to provide stable level sensing under a wide  
variety of changing conditions. Only a single external  
capacitor is required for operation.  
To 10x Scope Probe  
Vcc  
1
2MΩ  
(optional)  
2
3
4
7
6
OUT1 SNS2  
OUT2 SNS1  
OUT 1  
OUT 2  
To Electrode(s)  
C
s
Figure 1-1 shows a basic QT114 circuit using the device, with  
conventional OUT drives and power supply connections. The  
sensing electrode can be connected to a single-tier or 2-tier  
electrode as required.  
1MΩ  
multi-turn  
pot (optional)  
5
FILT  
POL  
Gnd  
8
Calibration is done by design, through adjustment of the  
electrode sizes and the Cs capacitor. Only under rare  
situations do QT114 circuits require calibration on an  
individual basis, and the circuit can make provision for that.  
V
dd  
V
dd  
FILTER  
POLARITY  
POL: 1 = Active High  
FILT: 1 = Slosh Filter  
1 SIGNAL ACQUISITION  
Figure 1-1 Standard mode options  
The QT114 employs a short, low duty cycle burst of  
charge-transfer cycles to acquire its signal. Burst mode  
permits power consumption in the low microamp range,  
dramatically reduces RF emissions, lowers susceptibility to  
EMI, and yet permits excellent response time. Internally the  
signals are digitally processed to generate the required  
output signals.  
It is not necessary to use both detection threshold points; if  
only single point sensing is desired, only the lower threshold  
and OUT1 can be used, while ignoring OUT2.  
Two option pins allow the selection of output polarity and the  
insertion of a 'slosh filter' before the OUT pins, as shown in  
Figure 1-1.  
The QT switches and charge measurement hardware  
functions are all internal to the QT114 (Figure 1-2). A 14-bit  
single-slope switched capacitor ADC includes both the  
required QT charge and transfer switches in a configuration  
that provides direct ADC conversion. The burst length is  
inversely proportional to the rate of charge buildup on Cs,  
which in turn depends on the values of Cs, Cx, and Vcc. Vcc  
is used as the charge reference voltage. Larger values of Cx  
cause the charge transferred into Cs to accumulate more  
rapidly. The trip points of the sensor can be changed by  
altering Cs and Cx, the load capacitance. As a result, the  
values of Cs, Cx, and Vcc should be fairly stable over the  
expected operating temperature range.  
1.1 ELECTRODE DRIVE  
The internal ADC treats Cs as a floating transfer capacitor; as  
a direct result, the sense electrode can be connected to  
either SNS1 or SNS2 with no performance difference. The  
polarity of the charge buildup across Cs during a burst is the  
same in either case. Cs must be of within a certain range for  
proper operation.  
It is possible to connect separate Cx and Cx’ loads to SNS1  
and SNS2 simultaneously, although the result is no different  
than if the loads were connected together at SNS2 (or  
SNS1). It is important to limit the amount of stray capacitance  
on both terminals, especially if the load Cx is already large,  
for example by minimizing trace lengths and widths so as not  
to exceed the Cx load specification and to allow for a larger  
sensing electrode size if so desired.  
Two fixed thresholds are used, one for low fluid level and the  
other for high level; adjusting Cs and Cx to allow these to trip  
at appropriate points is required by design, and if required  
may be trimmed by an adjustment. Figure 1-1 shows the  
optional potentiometer which can be used to fine-tune the  
placement of these threshold points relative to the signal.  
The PCB traces, wiring, and any components associated with  
or in contact with SNS1 and SNS2 will become proximity  
sensitive and should be treated with caution.  
1.2 THRESHOLD POINTS  
The QT114 employs twin threshold points set at both  
250 (for T1) and 150 counts (for T2) of acquisition  
signal. The signal travels in an inverse direction:  
increasing amounts of Cx reduce the signal level; the  
baseline ('dry') signal should lie at 300 counts or more  
under most conditions. Calibration details are  
discussed fully in Section 3.2.  
ELECTRODE  
Result  
SNS2  
Cs  
Start  
Cx  
2 ELECTRODE DESIGN  
Done  
The QT114 is designed to operate with a 'plateau'  
sensor, having a substantial surface area at each  
desired trip point, to create a capacitive 'step'.  
SNS1  
Charge  
Amp  
Figure 1-2 Internal Switching and Timing  
LQ  
2
QT114 R1.04/1106  
As Figure 2-1 shows, a vertical strip sensor on the outside  
of a container (or a vertical, insulated rod in the fluid) will  
generate a long sloping signal. The desired trip point 'T' is  
subject to a great deal of variation in location if the sensing  
signal drifts much, for example due to changes in Cs or Cx  
over the operating temperature range.  
Signal  
l2  
T1  
Figure 2-2 shows the response from a horizontal strip of  
the same surface area; the signal exhibits a very rapid rise  
in signal between points l1 and l2. Variations in circuit gain  
or signal drift have much less of an effect on the trip point  
with this orientation.  
T1  
l1  
Level  
l2  
l1  
In some cases (thin walled vessels for example) it may be  
sufficient to have a small round or square electrode patch  
on the exterior.  
Figure 2-1 Signal vs. Level for an External Vertical Strip  
Figure 2-3 shows the response from a twin-level external  
electrode set. The use of two horizontal electrode planes  
or tiers creates well-defined trip points that can be used to  
sense both 'low' and 'high' levels. A crossing of threshold  
T1 will be reflected in the OUT1 signal, while T2 will be  
reflected on OUT2.  
Signal  
l2  
2.1 EXTERNAL ELECTRODES  
T1  
l1  
External electrodes should be electrically conductive; metal  
foils and conductive carbon are both possible. Care should  
be taken that other objects or people near the vessel will  
not touch the electrode; in some cases shielding around  
the electrode with grounded metal will be required to  
prevent disturbances. If used, the shield element should be  
spaced apart from the electrode by an air gap or a  
low-density foam to reduce Cx loading.  
T1  
Level  
l1  
l2  
Figure 2-2 Signal vs. Level for an External Horizontal Strip  
Signal  
The required surface area of the external electrode will  
depend on the amount of signal needed to bracket the  
detection threshold, which in turn will depend in part on Cs  
and stray Cx. External electrodes sensing through thick  
walls and/or sensing low permittivity fluids will require  
larger surface areas than those sensing water through thin  
plastic, for example. External electrodes are more likely to  
require potentiometer trimming to achieve reliable  
operation (Figure 1-1, also Section 3.2).  
T2  
l4  
T2  
l3  
T1  
l2  
T1  
l1  
Level  
Note that external electrodes used with conductive  
solutions (i.e. aqueous liquids) do not measure the  
permittivity of the fluid: they actually measure the  
permittivity of the vessel wall, between 2 plates: the  
electrode (plate 1) and the fluid (plate 2, effectively a  
variable-area ground plate): if the fluid were to be replaced  
with mercury the signal would be unchanged. A 20%  
thickness variation in the vessel wall will therefore introduce  
about a 20% variation in the resulting capacitance; if the  
vessel wall cannot be controlled accurately enough in  
production, serious sensing errors may occur.  
l3 l4  
l1  
l2  
Figure 2-3 Signal vs. Level for Twin Horizontal Strips  
Numerous types of internal point-level probes are possible.  
2.2.1 DISC  
P
ROBES  
The simplest internal geometry is probably a disc probe  
(Figure 2-4), having at least one planar surface ('tier') parallel  
to the fluid surface. The sensing error can be minimized by  
making the tier thin, so that the signal transitions abruptly  
higher (see Figure 2-2) as the fluid covers the tier.  
When external electrodes are used to sense non-aqueous  
substances (like oils or gasoline), the vessel wall dielectric  
becomes a lessor contributor to the overall signal, which is  
then heavily dominated by the permittivity of the fluid. The  
lower the permittivity of the fluid the greater its dominance.  
A notable difficulty with disc probes is the task of insulating  
them with a uniform, repeatable thickness of insulation.  
2.2.2 SPIRAL  
W
IRE  
PROBES  
A spiral solid-wire probe is simple to construct (Figure 2-5),  
and has the advantage of being pre-insulated in a wide  
choice of plastics from inexpensive PVC to PTFE. These  
probe types provide a large step-function of capacitance  
localized at the desired trip point, and are easy to form.  
2.2 INTERNAL PROBES  
When used with aqueous fluids or other electrically  
conducting liquids, internal probes should be insulated with a  
plastic layer. See also Section 2.1 for a discussion of  
electrodes when used with conductive fluids. Aqueous  
probes should be 100% insulated, even on the cut end of a  
wire probe. The slightest pinhole of exposed metal anywhere  
on an immersed part of the probe will immediately convert  
the probe into a bare-metal probe (see Section 2.2.5).  
Spiral wire probes are most effective in water-based fluids;  
they are not as effective in oils and other nonconductive  
substances.  
LQ  
3
QT114 R1.04/1106  
Figure 2-4 Single Level Internal Planar Probe  
Figure 2-5 Single Level Internal Spiral Wire Probe  
T2  
T2  
Figure 2-6 Twin-Level Internal Planar Probe  
Figure 2-7 Twin-Level Internal Spiral Wire Probe  
Spiral wire probes have the disadvantage of not being as Coaxial probes are more expensive to make, and can have  
rugged as a solid disc probe.  
problems with vibration if they are not constructed robustly.  
The outer cylinder should be perforated at key spots to allow  
fluid to fill and drain the cavity without trapping air bubbles  
inside. The outer cylinder can also be made of a wire mesh.  
2.2.3 SIDE-ENTRY  
PROBES  
Another type is a side-entry probe (Figure 2-8), which  
requires an entry point into the vessel wall, but may have the  
advantage of accessibility in certain cases. These can be  
made of simple metal rod, insulated in almost any plastic if  
required.  
The outer cylinder does not have to be coated in plastic, even  
when used with water-based fluids. When used with oils, the  
inner rod does not require insulation either.  
2.2.5 BARE  
M
ETAL  
PROBES  
2.2.4 COAXIAL  
PROBES  
Bare metal internal probes can be used, for example with  
nonconductive fluids like oils, without difficulty. This applies  
to all probe types described above.  
Another type of internal probe is the coaxial probe (Figure  
2-10); these are most useful with oils or similar fluids having  
a low dielectric constant; the inner rod is connected to the  
signal connection, and together with the outer grounded  
cylinder forms a capacitor whose dielectric is either air or oil.  
Keeping the gap between rod and cylinder to a minimum  
increases the 'gain' of the electrode.  
Bare probes can also be used with aqueous fluids, but in  
these cases a 1,000pF (1nF) ceramic NPO capacitor should  
be inserted between the probe and the QT114 to block DC  
current flows.  
LQ  
4
QT114 R1.04/1106  
T1  
Figure 2-9 Twin-Level Internal Horizontal Probes  
Figure 2-8 Single Internal Horizontal Probe  
T2  
Figure 2-10 Coaxial Probe For Non-Aqueous Fluids  
Figure 2-11 Twin-Level Coaxial Probes For  
Non-Aqueous Fluids  
A bare internal probe used with conductive fluids and an  
in-line blocking capacitor will generate a huge, robust  
capacitive response that will not readily permit the use of a  
two-level probe due to signal saturation. Even the slightest  
amount of bare metal exposed to the fluid will usually  
generate an immediate, large response with aqueous fluids.  
2.2.7 VISCOUS, CONDUCTIVE  
F
ILMS  
Highly viscous fluids, or those having a high surface tension,  
and having substantial conductivity can fool some electrode  
designs into thinking that there is fluid present when there is  
not. This is a particular problem with external electrodes,  
where the residual films of certain types of fluids inside the  
container, electrically coupled to the fluid mass below, will  
create a substantial capacitive response. Internal probes are  
much more resistant to this effect since the fluid surface is  
guaranteed to become mechanically disconnected from the  
probe when the level drops. Coating the inner vessel surface  
with a smooth plastic of polyethylene or PTFE often has a  
very beneficial effect on this phenomenon.  
2.2.6 SCALE  
B
UILDUP  
Scale buildup on internal probes, bare or insulated, is not  
generally a problem since the sensor is still measuring  
capacitance, not conductance, and  
a
reduction in  
conductivity around the probe will have minimal or no effect.  
Probe designs should be tested for this to be certain in all  
specific cases.  
2.3 SINGLE LEVEL SENSING  
A legitimate concern with bare metal probes is the buildup of  
scale or other deposits at the entry point of the probe into the  
vessel. Such deposits may create a conductive surface path  
(especially if the vessel is made of metal) that may lead to  
false-positive trips. If the shank of the probe at the entry point  
is insulated enough so that conductive bridging cannot occur,  
this problem should be alleviated.  
When sensing for a single trip point, the single electrode can  
be a simple horizontal strip on the outside of a nonmetallic  
vessel (Figure 2-2), or an internal probe having a substantial  
horizontal 'plateau' at the trip point (Figures 2-4, 2-5, 2-8,  
2-10). When the strip or plateau is ‘covered’ with fluid the IC  
will detect on at least the OUT1 line; OUT2 can be ignored.  
LQ  
5
QT114 R1.04/1106  
The trip point ideally occurs at the  
centerline of the internal probe or external  
electrode; this can be trimmed with a  
In the case of coaxial probes, the ground  
connection is inherent in the outer cylinder  
and no further ground connection is  
required.  
potentiometer  
if  
necessary  
(see  
Section 3.2). Making the electrode narrow  
and long (horizontally) will help keep the  
trip point localized within a narrow band.  
3 PROCESSING AND  
CIRCUITRY  
2.4 DUAL LEVEL SENSING  
When two trip levels are desired, for  
example for high-low limit sensing, the  
electrode or probe set should have two  
3.1 SLOSH FILTER  
It is desirable to suppress rapid, multiple  
detections of fluid level generated by the  
surface movement of the fluid, for example  
in a moving vehicle. To accomplish this,  
distinct tiers.  
A typical twin external  
electrode is shown in Figure 2-3 (they are  
connected together to the sense line);  
typical internal twin electrodes are shown  
in Figures 2-6, 2-7, 2-9, and 2-11. The  
response of a properly constructed 2-tier  
probe is shown in Figure 2-3.  
the QT114 incorporates  
a
detection  
integration counter that increments with  
each detection until a limit is reached, after  
which point one of the OUT lines is  
activated. If during a detection ‘event’ the  
fluid level falls below the electrode level  
(signal rises above a 'T' point in signal  
counts), the counter decrements back  
towards zero. Over a long interval the up  
and down counts will tend towards either  
zero or the limit, with the result being a  
statistical function of the number of  
detections vs. nondetections. If on average  
there are more detections than nondetections, the counter  
will eventually make its way to the limit value and an OUT line  
will activate.  
Dual level electrodes should have an  
approximately 3:1 surface area ratio or  
more from T2 to T1; that is, the surface  
area at T2 should be at least 3x the  
surface area of the electrode at T1. There  
is no penalty for making T2 excessively  
large. The high ratio is required to  
Figure 2-12 A two-tier spiral wire  
probe with ground rod  
overcome the QT114's decreasing gain with increasing Cx  
load (Figures 4-1, 4-2).  
With internal dual-level probes where T1 and T2 are  
substantially separated, the intervening connection between  
the two levels should be more thickly insulated, for example  
with a thick plastic spacer, and any remaining internal gap  
inside the spacer should be filled with silicone sealant or  
epoxy. This will help to prevent the signal from rising much  
between the two levels, thus preserving a crisp bi-level  
response like that shown in Figure 2-3.  
Once a detection has been established, the counter must find  
its way back to zero before the affected OUT line goes  
inactive, via the same process. Although the counter has a  
nominal reaction time of 15 seconds, in some cases it may  
take several minutes before the outcome is resolved  
depending on the violence of the fluid surface. If the fluid  
surface is stable however, it will only require 15 seconds to  
change the state of an OUT line.  
2.5 GROUNDING CONSIDERATIONS  
In all cases ground reference coupling to the fluid must be  
made. In aqueous fluids, this can simply mean connecting  
the metal vessel to circuit ground, or inserting a bare metal  
element into the bottom of a plastic or glass vessel. The  
degree of galvanic contact is not critical, so scale and  
corrosion on the ground electrode are not of great concern  
especially if the 'connection' to the fluid is substantial enough .  
Both OUT1 and OUT2 have their own independent slosh  
filters. Both are enabled or disabled in unison by strap option,  
pin 4, 'FILT' as follows:  
FILT = Gnd  
FILT = Vcc  
Slosh filter off  
Slosh filter on  
FILT strapping can be changed 'on the fly'.  
If direct electrical contact to the fluid is not possible, a large  
piece of external metal can be bonded to the outside of the  
vessel and grounded. Once this is done, the signal should be  
monitored while the vessel is touched by hand; if the  
grounding is sufficient, the signal will not move or will move  
only slightly.  
3.2 CALIBRATION  
Both the T1 and T2 trip point values are hardwired internally  
as functions of counts of burst length. Sensitivity can be  
altered relative to these trip points by altering electrode size,  
geometry, degree of coupling to the fluid, and the value of  
Cs. Selecting an appropriate value of Cs for a given electrode  
geometry is essential for solid detection stability.  
Very large vessels, even if not grounded, often do not require  
additional provision for grounding since the bottom surface  
area and free-space capacitance of the tank may be  
sufficient for ground return coupling.  
The QT114 employs dual threshold points set at 250 and 150  
counts of acquisition signal. The signal travels in a reverse  
direction: increasing Cx reduces the signal counts; as a  
result, 250 counts of signal corresponds to the most sensitive  
or ‘lower’ setting (T1), and 150 the least sensitive 'upper'  
setting (T2).  
In some cases (windshield washer tanks on cars for  
example) there will exist a water path to a chassis-grounded  
fitting somewhere downstream of the tank, or the water path  
may be labyrinthine enough to provide enough capacitive  
coupling to the grounded chassis even if it does not make  
galvanic contact. In these cases no further provision for fluid  
grounding is required. Simple experimentation will easily  
determine whether the existing amount of parasitic coupling  
to ground is enough to do the job.  
LQ  
6
QT114 R1.04/1106  
The baseline signal count when the electrodes are 'dry'  
should begin at over 300 counts or more if possible. With a  
small, weakly coupled electrode the baseline signal can be  
trimmed to be closer to the 250 mark with a potentiometer to  
provide a higher apparent gain by closing the gap between  
the baseline and T1 (see below). The spread between T2 and  
T1 is fixed and cannot be separately trimmed.  
3.3 INTERFACING  
3.3.1 OUT LINES AND  
P
OLARITY  
SELECTION  
The QT114 has two OUT pins, OUT1 and OUT2, which  
correspond to the crossings of signal at T1 and T2  
respectively. Each output will become active after the  
threshold is crossed, and after the slosh filter (if enabled) has  
settled to its final state. The polarity of the OUT lines is  
Increasing Cs will increase the baseline counts, while determined by pin 5, 'POL', as follows:  
increasing Cx will decrease it. When optimally tuned, each  
POL = Gnd  
POL = Vcc  
Outputs active low  
Outputs active high  
threshold point will be symmetrically bracketed by signal  
swing, with an intermediate count at about 200 between the  
two. Thus, the lower electrode level should cause a signal There is no timeout on these outputs; the OUT lines will  
swing that (when 'dry') starts at 300 or more and when remain active for as long as the thresholds are crossed.  
covered ends at about 200. The upper electrode when  
The OUT lines can sink up to 5mA of non-inductive current. If  
covered should generate a signal level of 100 or less.  
an inductive load is used, like a small relay, the load should  
be diode clamped to prevent device damage.  
There is a hysteresis of 3 counts around both T1 and T2.  
The signal can be viewed for setup purposes with an  
oscilloscope via a 10x or FET probe connected to a 2M ohm  
resistor as shown in Figure 1-1; the resistor is required to  
reduce the loading effect of the scope probe capacitance.  
When viewed this way the signal will appear as a declining  
slope (Figure 3-1). The duration of the slope corresponds to  
the burst length: each count of burst takes approximately 7  
microseconds on average. The ‘low level’ threshold at 250  
counts is at 1750 microseconds from the start of the  
waveform, while the 150 count ‘upper’ threshold is at about  
1050 microseconds from the start, at 3 volts Vcc. These trip  
points can be easily observed by monitoring the OUT lines  
while watching the signal on a scope, by increasing Cx  
loading until each OUT line activates in turn. FILT should be  
off to speed up response during testing.  
POL strapping can be changed 'on the fly'.  
Cycling and Stiction: Care should be taken when the QT114  
and the loads are powered from the same supply, and the  
supply is minimally regulated. The QT114 derives its internal  
references from the power supply, and sensitivity shifts can  
occur with changes in Vcc, as happens when loads are  
switched on. This can induce detection ‘cycling’, whereby a  
trip point is crossed, the load is turned on, the supply sags,  
the trip is no longer sensed, the load is turned off, the supply  
rises and the trip point is reacquired, ad infinitum. To prevent  
this occurrence, the outputs should only be lightly loaded if  
the device is operated from a poorly regulated supply.  
Detection ‘stiction’, the opposite effect, can occur if a load is  
shed when an Out line becomes active.  
3.3.2 HEARTBEAT™ OUTPUT  
The QT114's internal clock is dependent on Vcc; as a result,  
the threshold points in terms of delay time from the start of  
the burst are also substantially dependent on Vcc, but they  
are always fixed in terms of signal counts. A regulated power  
supply is strongly advised to maintain the proper calibration  
points.  
Both OUT lines have a full-time HeartBeat™ ‘health’ indicator  
superimposed on them. These operate by taking both OUT  
pins into a 3-state mode for 350µs once before every QT  
measurement burst. This state can be used to determine that  
the sensor is operating properly, or, it can be ignored using  
one of several simple methods.  
Potentiometer adjustment: The external potentiometer shown  
in Figure 1-1 is optional and in most cases not required. In  
situations where the electrode pickup signal is weak,  
trimming may be necessary on a production basis to make  
the device sensitive enough. Trimming affects the baseline  
reference of the signal, and thus effects the amount of  
change in the signal required to cause a threshold crossing.  
If active-low polarity is selected, the HeartBeat indicator can  
be sampled by using a pulldown resistor on one or both OUT  
lines, and feeding the resulting negative-going pulse(s) into a  
counter, flip flop, one-shot, or other circuit (Figure 3-2). In this  
configuration, the pulldown resistor will create negative-going  
HeartBeat pulses when the sensor is not detecting fluid;  
when detecting fluid, the OUT line will remain low for the  
duration of the detection, and no pulse will be evident.  
Potentiometer trimming is not a substitute for a good choice  
of Cs. In low signal situations Cs should still be  
determined by design to allow the baseline signal to be  
just beyond T1 as viewed on a scope. The trimmer should  
then be added and the baseline adjusted to the necessary  
final resting point.  
The trimmer should never be adjusted so that the  
resistance from ground to SNS1 or SNS2 is less than  
200K ohms. If the resistance is less than this amount, the  
gain of the circuit will be appreciably reduced and it may  
stop functioning altogether. A 200K resistor from the wiper  
to ground can be added to limit trim current at the  
extremes of wiper travel.  
Figure 3-1 Burst Waveform at 2M Pickoff Resistor  
LQ  
7
QT114 R1.04/1106  
Figure 3-2  
Figure 3-3  
Getting HeartBeat pulses with a pull-down resistor  
Using a micro to obtain HB pulses in either output state  
HeartBeat™ Pulses  
2
7
6
5
PORT_M.1  
2
3
4
7
6
5
OUT1  
OUT2  
FILT  
SNS2  
SNS1  
POL  
OUT1  
OUT2  
FILT  
SNS2  
SNS1  
POL  
R
R
1
2
PORT_M.2  
PORT_M.3  
Ro  
3
4
Microprocessor  
Ro  
PORT_M.4  
Conversely, a pull-up resistor will show HeartBeat pulses The QT114 does have diode protection on its terminals which  
when the line is low (detecting).  
can absorb and protect the device from most induced  
discharges, up to 20mA; the usefulness of the internal  
clamping will depending on the probe insulation's dielectric  
properties, thickness, and the rise time of the transients. ESD  
dissipation can be aided further with an added diode  
protection network as shown in Figure 3-5. Because the  
If active-high OUT polarity is selected, the pulses will only  
appear if there is a pull-up resistor in place and the fluid is  
not present (no detection, low output), or, if there is a  
pull-down resistor and the output is active (high output).  
If the sensor is wired to a microprocessor as shown in Figure charge and transfer times of the QT114 are relatively long,  
the circuit can tolerate very large values of Re1, as much as  
50k ohms in most cases without affecting gain. The added  
3-3, the microprocessor can reconfigure the load resistor to  
either ground or Vcc depending on the output state of the  
QT114, so that the pulses are evident in either state with diodes shown (1N4150, BAV99 or equivalent low-C diodes)  
will shunt the ESD transients away from the part, and Re1 will  
current-limit the rest into the QT110's own internal clamp  
diodes. C1 should be around 10µF if it is to absorb positive  
transients from a human body model standpoint without  
rising in value by more than 1 volt. If desired C1 can be  
replaced with an appropriate zener diode. Directly placing  
semiconductor transient protection devices or MOV's on the  
sense lead is not advised; these devices have extremely  
large amounts of parasitic C which will swamp the sensor.  
either POL setting.  
GATE OR  
MICRO INPUT  
2
3
4
7
6
5
CMOS  
OUT1  
OUT2  
FILT  
SNS2  
SNS1  
POL  
Co  
100pF  
CMOS  
Re2 functions to isolate the transient from the QT110's Vcc  
pin; values of around 1K ohms are reasonable.  
100pF  
Co  
As with all ESD protection networks, it is important that the  
transients be led away from the circuit. PCB ground layout is  
crucial; the ground connections to the diodes and C1 should  
all go back to the power supply ground or preferably, if  
available, a chassis ground connected to earth. The currents  
should not be allowed to traverse the area directly under the  
QT114.  
Figure 3-4 Eliminating HB Pulses  
Electromechanical devices will ignore this short pulse. The  
pulse also has too low a duty cycle to visibly affect LED’s. It  
can be filtered completely if desired, by adding an RC time  
constant to filter the output, or if interfacing directly and only  
to a high-impedance CMOS input, by doing nothing or at  
most adding a small noncritical capacitor from each used  
OUT line to ground (Figure 3-4).  
Vcc  
C 10✙F  
1
Re  
Re  
2
1
3.4 ESD PROTECTION  
In some installations the QT114 will be protected from direct  
static discharge by the insulation of the electrode and the  
fact that the probe may not be accessible to human contact.  
1
To Electrodes  
2
3
4
7
6
5
OUT1  
OUT2  
FILT  
SNS2  
SNS1  
POL  
However, even with probe insulation, transients can still flow  
into the electrode via induction, or in extreme cases, via  
dielectric breakdown. Some moving fluids (like oils) and  
powders can build up a substantial triboelectric charge  
directly on the probe surface.  
C
S
8 Gnd  
Figure 3-5 ESD Protection Network  
LQ  
8
QT114 R1.04/1106  
If the QT114 is connected to an external circuit via a long Supply drain can be calculated from the adjusted voltage  
cable, it is possible for ground-bounce to cause damage to droop using the basic charge equation:  
the OUT pins; even though the transients are led away from  
the QT114 itself, the connected signal or power ground line  
will act as an inductor, causing a high differential voltage to  
build up on the OUT wires with respect to ground. If this is a  
possibility, the OUT pins should have a resistance in series  
with them on the sensor PCB to limit current; this resistor  
should be as large as can be tolerated by the load.  
✁VC  
i =  
t
where C is the supply capacitor's value, t is the elapsed  
measurement time in seconds, and DV is the adjusted  
voltage droop on C.  
3.7 PC BOARD LAYOUT  
3.5 SAMPLE CAPACITOR  
There are only a few important issues for the PCB layout. For  
RF susceptibility reasons it should be compact, and if  
possible use SMT components and a ground plane (Section  
3.8). Lines for SNS1 and SNS2 should be short and not run  
directly over the ground plane to reduce Cx loading, which  
adversely affects sensitivity (Section 3.2). ESD issues should  
be taken into account (Section 3.4). The board should not be  
located in a place where there are wild temperature swings  
which can cause excessive drift in Cs. The voltage regulator  
should be located nearby and should only be shared with  
other circuits that do not induce supply sags or spikes  
(Section 3.6).  
Charge sampler Cs should be a stable grade of capacitor,  
like PPS film, NPO ceramic, or polycarbonate. The  
acceptable Cs range is anywhere from 10nF to 100nF  
(0.1uF) and its required value will depend on load Cx. In  
some cases, to achieve the 'right' value, two or more  
capacitors may need to be wired in parallel.  
The value of Cs controls the calibration point (Section 3.2)  
and its selection should not be taken lightly.  
3.6 POWER SUPPLY  
The power supply can range from 2.5 to 5.0 volts. At 3 volts  
current drain averages less than 20µA in most cases.  
Operation can be from batteries, especially stable Lithium  
cells, but be cautious about loads causing supply droop  
(Section 3.3.1).  
3.8 RFI / EMI ISSUES  
3.8.1 SUSCEPTIBILITY  
The QT114 is remarkably resistant to RF fields. With enough  
field strength at frequencies above 100MHz, internal  
protection diode conduction at the SNS1 and SNS2 pins can  
occur and destroy the charge-transfer process, causing false  
detections or desensitization, or alternating cycles of both.  
If the power supply is shared with another electronic system,  
care should be taken to assure that the supply is free of  
digital spikes, sags, and surges which can adversely affect  
the QT114.  
Susceptibility can be dramatically reduced by adding a  
resistor in series with the Sense line, between 2K to 60K  
ohms depending on load Cx. This has the effect of creating a  
natural low-pass filter in conjunction with the Cs capacitor to  
filter out external RF components. If an ESD network is used  
(Figure 3-5), the added resistor should be placed between  
the clamp diodes and the sense probe, and Re1 should be  
made very small, 1K ohms or less, or even eliminated. With a  
50pF load the added resistance should be no greater than  
about 5.6K ohms, while at 10pF it can be as high as 27K; the  
value should be chosen to allow at least 7 RC time constants  
of settling with a 2µs charge time for efficient, stable  
operation. 5% tolerance resistors can be used.  
If desired, the supply can be regulated using a conventional  
low current regulator, for example CMOS regulators that have  
nanoamp quiescent currents. The voltage regulator should  
not have a minimum load specification, which almost  
certainly will be violated by the QT114's low current  
requirement.  
Since the QT114 operates in a burst mode, almost all the  
power is consumed during the course of each burst. During  
the time between bursts the sensor is quiescent.  
3.6.1 MEASURING  
SUPPLY  
C
URRENT  
Measuring average power consumption is a fairly difficult  
task, due to the burst nature of the QT110's operation. Even  
a good quality RMS DMM will have difficulty tracking the low  
burst rate.  
A great number of susceptibility problems can be traced to  
RF fields coupling directly to components on the PCB.  
Therefore a shielded, grounded housing is recommended to  
reduce susceptibility. The use of SMT circuitry is also highly  
recommended; physically reducing lead lengths of the wiring  
traces and pins, along with a poured-copper ground plane,  
will dramatically reduce the coupling of external RF fields.  
The simplest method for measuring average current is to  
replace the power supply with a large value low-leakage  
electrolytic capacitor, for example 2,700µF. 'Soak' the  
capacitor by connecting it to a bench supply at the desired  
operating voltage for 24 hours to form the electrolyte and  
reduce leakage to a minimum. Connect the capacitor to the  
QT114 circuit at T=0, making sure there will be no detections  
during the measurement interval and no loads on the OUT  
pins; at T=30 seconds measure the capacitor's voltage with a  
DMM. Repeat the test without a load to measure the  
capacitor's internal leakage, and subtract the internal leakage  
result from the voltage droop measured during the QT114  
load test. Be sure the DMM is connected only at the end of  
each test, to prevent the DMM's own impedance from  
contributing to the capacitor's discharge.  
3.8.2 RF EMISSIONS  
RF emissions are extremely weak, as the charge-transfer  
pulse frequency is only about 170kHz and the bursts are  
sparsely spaced, so that the average spectral power density  
is extremely low. The addition of a series resistor for EMI  
reasons (above) will dramatically reduce edge rise and fall  
times, resulting in an even greater reduction in emitted RF  
energy.  
LQ  
9
QT114 R1.04/1106  
4.1 ABSOLUTE MAXIMUM SPECIFICATIONS  
Operating temp. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . as designated by suffix  
Storage temp. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . -55OC to +125OC  
V
CC. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . -0.5 to +6.5V  
Max continuous pin current, any control or drive pin . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ±20mA  
Short circuit duration to ground, any pin. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . infinite  
Short circuit duration to VCC, any pin. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . infinite  
Voltage forced onto any pin. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . -0.6V to (Vcc + 0.6) Volts  
4.2 RECOMMENDED OPERATING CONDITIONS  
V
CC. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . +2.5 to 5.25V  
Supply ripple+noise. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20mV p-p max  
Load capacitance. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 0 to 50pF  
Cs value. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10nF to 100nF  
4.3 AC SPECIFICATIONS Vcc = 3.0, Ta = recommended operating range  
Parameter  
Notes  
Description  
Min  
Typ  
Max  
Units  
T
PC  
Charge duration  
2
4
µs  
µs  
T
T
T
PT  
BS  
BL  
Transfer duration  
Burst spacing interval  
Burst length, time  
75  
ms  
ms  
counts  
ms  
µs  
0
6
3.5  
14  
N
LC  
Maximum burst length counts  
Response time  
500  
80  
T
R
Slosh filter disabled  
T
Q
QT pulse spacing  
T
R
Pulse edge risetime  
Pulse edge falltime  
Heartbeat pulse width  
6
4
ns  
Cx = 5pF  
Cx = 5pF  
T
F
ns  
T
HB  
300  
350  
400  
µs  
4.4 SIGNAL PROCESSING  
Description  
Typ  
Notes  
Min  
Max  
Units  
Threshold, T1  
250  
150  
3
counts  
counts  
counts  
seconds  
counts  
Note 1  
Note 1  
Note 1  
Note 2  
Note 1  
Threshold, T2  
Hysteresis  
Slosh filter length, time  
Slosh filter length, counts  
15  
190  
Note 1: Counts of burst  
Note 2: Uninterrupted detection / non-detection: Strap option.  
LQ  
10  
QT114 R1.04/1106  
4.5 DC SPECIFICATIONS  
Vcc = 3.0V, Cs = 10nF, Cx = 10pF, T = recommended range, unless otherwise noted  
A
Parameter  
Notes  
Description  
Min  
Typ  
Max  
Units  
V
CC  
DD  
DDS  
Supply voltage  
2.45  
5.25  
V
I
Supply current  
20  
µA  
V
Supply turn-on slope  
Low input logic level  
High input logic level  
Low output voltage  
High output voltage  
Input leakage current  
Load capacitance range  
Sensitivity [T1]  
100  
2.2  
V/s  
Required for proper startup  
FILT, POL  
V
IL  
0.8  
0.6  
V
V
HL  
V
FILT, POL  
V
OL  
V
V
OUT1, OUT2, 5mA sink  
OUT1, OUT2, 1mA source  
FILT, POL  
V
OH  
IL  
Vdd-0.8  
0
I
±1  
50  
µA  
C
X
pF  
S[1]  
S[2]  
0.125  
0.33  
pF/count  
pF/count  
Cs = 20nF, Cx = 10pF  
Cs = 20nF, Cx = 30pF  
Sensitivity [T2]  
FIGURE 4-1  
FIGURE 4-2  
Gain vs. Cx Load; Cs = 20nF; Vcc = 3.0  
Signal Level vs. Cx Load; Cs = 20nF; Vcc = 3.0  
18  
16  
14  
12  
10  
8
400  
350  
300  
250  
200  
150  
100  
50  
6
4
2
0
0
0.00  
10.00  
20.00  
30.00  
40.00  
0.00  
10.00  
20.00  
30.00  
40.00  
Cx Load, pF  
Cx Load, pF  
LQ  
11  
QT114 R1.04/1106  
4.6 MECHANICAL  
Package type: 8-pin Dual-In-Line  
Millimeters  
Inches  
Max  
SYMBOL  
Min  
Max  
Notes  
Min  
Notes  
a
A
6.096  
7.62  
7.112  
8.255  
10.922  
7.62  
-
0.24  
0.3  
0.28  
0.325  
0.43  
0.3  
M
m
Q
P
9.017  
7.62  
Typical  
BSC  
0.355  
0.3  
Typical  
BSC  
0.889  
0.254  
0.355  
1.397  
2.489  
3.048  
0.381  
3.048  
-
0.035  
0.01  
0.014  
0.055  
0.098  
0.12  
0.015  
0.12  
-
-
-
-
L
0.559  
1.651  
2.591  
3.81  
-
0.022  
0.065  
0.102  
0.15  
-
L1  
F
Typical  
BSC  
Typical  
BSC  
R
r
S
3.556  
4.064  
7.062  
9.906  
0.381  
0.14  
0.16  
0.3  
S1  
Aa  
x
7.62  
0.3  
8.128  
0.203  
0.32  
0.008  
0.39  
0.015  
Y
Package type: 8-pin SOIC  
Millimeters  
Inches  
Max  
SYMBOL  
Min  
Max  
Notes  
Min  
Notes  
M
W
Aa  
H
h
4.800  
5.816  
3.81  
4.979  
6.198  
3.988  
1.728  
0.762  
1.27  
0.189  
0.229  
0.15  
0.196  
0.244  
0.157  
0.068  
0.01  
0.05  
0.019  
0.04  
0.01  
0.03  
8º  
1.371  
0.101  
1.27  
0.054  
0.004  
0.050  
0.014  
0.02  
D
L
BSC  
BSC  
0.355  
0.508  
0.19  
0.483  
1.016  
0.249  
0.762  
8º  
E
e
0.007  
0.229  
0º  
ß
0.381  
0º  
Ø
LQ  
12  
QT114 R1.04/1106  
5 ORDERING INFORMATION  
PART  
TEMP RANGE  
PACKAGE  
MARKING  
PDIP  
Lead-Free  
SOIC-8  
Lead-Free  
QT114 or  
QT114-DG  
QT114 or  
QT1G  
QT114-DG  
0 - 70C  
QT114-ISG  
-40 - 85C  
6 MOISTURE SENSITIVITY LEVEL (MSL)  
MSL Rating  
Peak Body Temperature  
Specifications  
IPC/JEDEC J-STD-020C  
MSL1  
260OC  
LQ  
13  
QT114 R1.04/1106  
lQ  
Copyright © 1999-2006 QRG Ltd. All rights reserved  
Patented and patents pending  
Corporate Headquarters  
1 Mitchell Point  
Ensign Way, Hamble SO31 4RF  
Great Britain  
Tel: +44 (0)23 8056 5600 Fax: +44 (0)23 8045 3939  
www.qprox.com  
North America  
651 Holiday Drive Bldg. 5 / 300  
Pittsburgh, PA 15220 USA  
Tel: 412-391-7367 Fax: 412-291-1015  
The specifications set out in this document are subject to change without notice. All products sold and services supplied by QRG are subject  
to QRG’s Terms and Conditions of sale and services. QRG patents, trademarks and Terms and Conditions can be found online at  
http://www.qprox.com/about/legal.php. Numerous further patents are pending, one or more which may apply to this device or the applications  
thereof.  
QRG products are not suitable for medical (including lifesaving equipment), safety or mission critical applications or other similar purposes.  
Except as expressly set out in QRG's Terms and Conditions, no licenses to patents or other intellectual property of QRG (express or implied)  
are granted by QRG in connection with the sale of QRG products or provision of services. QRG will not be liable for customer product design  
and customers are entirely responsible for their products and applications which incorporate QRG's products.  
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