QProx™ QT114
CHARGE-TRANSFER QLEVEL™ SENSOR IC
Limit sensing of almost any fluid or powder
2-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
Vcc
1
Out1
2
Out2
3
Filt
4
QT114
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8
Gnd
7
Sns2
6
Sns1
5
Pol
APPLICATIONS !
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Process controls
Vending machines
Automotive fluids
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Consumer appliances
Medical fluid sensing
Soil moisture sensing
DESCRIPTION The QT114 QuickLevel™ 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.
TA
00C to +700C
-400C to +850C
Quantum Research Group Ltd
AVAILABLE OPTIONS
SOIC
QT114-S
QT114-IS
8-PIN DIP
QT114-D
Copyright © 1999 Quantum Research Group Ltd
R1.03
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
OUT 1
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.
4 FILT
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.
To Electrode(s)
Cs
1MΩ multi-turn
pot (optional)
POL 5
8 Gnd
Vdd
Vdd
FILTER
POLARITY
POL: 1 = Active High
FILT: 1 = Slosh Filter
1 - SIGNAL ACQUISITION
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.
3 OUT2 SNS1 6
OUT 2
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.
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.
2MΩ (optional)
2 OUT1 SNS2 7
Figure 1-1 Standard mode options
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.
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.
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
E LE C T RO DE
Result
Done
Single-Slo pe 14-bit
Switched Capacitor ADC
Bu rst Controller
Start
S NS 2
Cs
Cx
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.
2 ELECTRODE DESIGN
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'.
S NS 1
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,
C ha rg e
Amp
Figure 1-2 Internal Switching & Timing
-2-
for example due to changes in Cs or Cx over the operating
temperature range.
Sign a l
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.
l2
T1
In some cases (thin walled vessels for example) it may be
sufficient to have a small round or square electrode patch
on the exterior.
T1
l1
l2
l1
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.
Le ve l
Figure 2-1 Signal vs. Level for an External Vertical Strip
S ig na l
2.1 EXTERNAL ELECTRODES
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.
l1
l2
T1
T1
l1 l2
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).
Le ve l
Figure 2-2 Signal vs. Level for an External Horizontal Strip
Sign a l
T2
l4
l3
T2
Note that external electrodes used with conductive
solutions (i.e. aqueous liquids) do not measure the
T1
l
T1
permittivity of the fluid: they actually measure the
l1 2
permittivity of the vessel wall, between 2 plates: the
Le ve l
electrode (plate 1) and the fluid (plate 2, effectively a
l 3 l4
l1 l2
variable-area ground plate): if the fluid were to be replaced
with mercury the signal would be unchanged. A 20%
Figure 2-3 Signal vs. Level for Twin Horizontal Strips
thickness variation in the vessel wall will therefore
introduce about a 20% variation in the resulting
capacitance; if the vessel wall cannot be controlled 2.2.1 DISC PROBES
accurately enough in production, serious sensing errors may The simplest internal geometry is probably a disc probe
(Figure 2-4), having at least one planar surface ('tier')
occur.
parallel to the fluid surface. The sensing error can be
When external electrodes are used to sense non-aqueous minimized by making the tier thin, so that the signal
substances (like oils or gasoline), the vessel wall dielectric transitions abruptly higher (see Figure 2-2) as the fluid
becomes a lessor contributor to the overall signal, which is covers the tier.
then heavily dominated by the permittivity of the fluid. The
A notable difficulty with disc probes is the task of insulating
lower the permittivity of the fluid the greater its dominance.
them with a uniform, repeatable thickness of insulation.
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).
Numerous types of internal point-level probes are possible.
2.2.2 SPIRAL WIRE 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.
Spiral wire probes are most effective in water-based fluids;
they are not as effective in oils and other nonconductive
substances.
-3-
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
rugged as a solid disc probe.
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
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.
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.
2.2.4 COAXIAL PROBES
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.
Coaxial probes are more expensive to make, and can have
problems with vibration if they are not constructed robustly.
The outer cylinder should be perforated at key spots to allow
2.2.5 BARE METAL 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.
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.
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
2-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.
-4-
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
2.2.6 SCALE BUILDUP
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.
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.
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.
2.3 SINGLE LEVEL SENSING
2.2.7 VISCOUS, CONDUCTIVE FILMS
The trip point ideally occurs at the centerline of the internal
probe or external electrode; this can be trimmed with a
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.
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
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.
-5-
3 - PROCESSING &
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
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.
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, 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. non-detections. If
on average there are more detections than
non-detections, 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
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 Figure 2-12 A 2-tier spiral wire
levels should be more thickly insulated, for
probe with ground rod
example with a thick plastic spacer, and
Once a detection has been established,
any remaining internal gap inside the spacer should be filled
with silicone sealant or epoxy. This will help to prevent the the counter must find its way back to zero before the affected
signal from rising much between the two levels, thus OUT line goes inactive, via the same process. Although the
preserving a crisp bi-level response like that shown in Figure counter has a nominal reaction time of 15 seconds, in some
cases it may take several minutes before the outcome is
2-3.
resolved depending on the violence of the fluid surface. If the
2.5 GROUNDING CONSIDERATIONS
fluid surface is stable however, it will only require 15
In all cases ground reference coupling to the fluid must be seconds to change the state of an OUT line.
made. In aqueous fluids, this can simply mean connecting
Both OUT1 and OUT2 have their own independent slosh
the metal vessel to circuit ground, or inserting a bare metal filters. Both are enabled or disabled in unison by strap
element into the bottom of a plastic or glass vessel. The option, pin 4, 'FILT' as follows:
degree of galvanic contact is not critical, so scale and
FILT = Gnd
Slosh filter off
corrosion on the ground electrode are not of great concern
FILT = Vcc
Slosh filter on
especially if the 'connection' to the fluid is substantial
enough.
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 3.2 CALIBRATION
vessel and grounded. Once this is done, the signal should be Both the T1 and T2 trip point values are hardwired internally
monitored while the vessel is touched by hand; if the as functions of counts of burst length. Sensitivity can be
grounding is sufficient, the signal will not move or will move altered relative to these trip points by altering electrode size,
geometry, degree of coupling to the fluid, and the value of
only slightly.
Cs. Selecting an appropriate value of Cs for a given
Very large vessels, even if not grounded, often do not require electrode geometry is essential for solid detection stability.
additional provision for grounding since the bottom surface
area and free-space capacitance of the tank may be The QT114 employs dual threshold points set at 250 and
150 counts of acquisition signal. The signal travels in a
sufficient for ground return coupling.
reverse direction: increasing Cx reduces the signal counts;
In some cases (windshield washer tanks on cars for as a result, 250 counts of signal corresponds to the most
example) there will exist a water path to a chassis-grounded sensitive or ‘lower’ setting (T1), and 150 the least sensitive
fitting somewhere downstream of the tank, or the water path 'upper' setting (T2).
may be labyrinthine enough to provide enough capacitive
coupling to the grounded chassis even if it does not make The baseline signal count when the electrodes are 'dry'
galvanic contact. In these cases no further provision for fluid should begin at over 300 counts or more if possible. With a
grounding is required. Simple experimentation will easily small, weakly coupled electrode the baseline signal can be
determine whether the existing amount of parasitic coupling trimmed to be closer to the 250 mark with a potentiometer to
provide a higher apparent gain by closing the gap between
to ground is enough to do the job.
the baseline and T1 (see below). The spread between T2
In the case of coaxial probes, the ground connection is and T1 is fixed and cannot be separately trimmed.
inherent in the outer cylinder and no further ground
Increasing Cs will increase the baseline counts, while
connection is required.
increasing Cx will decrease it. When optimally tuned, each
threshold point will be symmetrically bracketed by signal
-6-
swing, with an intermediate count at about 200 between the
two. Thus, the lower electrode level should cause a signal
swing that (when 'dry') starts at 300 or more and when
covered ends at about 200. The upper electrode when
covered should generate a signal level of 100 or less.
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.
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.
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.
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.
The OUT lines can sink up to 5mA of non-inductive current.
If an inductive load is used, like a small relay, the load
should be diode clamped to prevent device damage.
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
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.
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. Conversely, a pull-up resistor will show HeartBeat
pulses when the line is low (detecting).
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
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
either POL setting.
3.3 INTERFACING
3.3.1 OUT LINES AND POLARITY 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 determined by pin 5, 'POL', as follows:
POL = Gnd
POL = Vcc
Outputs active low
Outputs active high
There is no timeout on these outputs; the OUT lines will
remain active for as long as the thresholds are crossed.
Figure 3-1 Burst Waveform at 2M Pickoff Resistor
-7-
Figure 3-2
Getting HeartBeat pulses with a pull-down resistor
Figure 3-3
Using a micro to obtain HB pulses in either output state
HeartBeat™ Pulses
2
OUT1
SNS2
7
PORT_M.1
6
Microprocessor PORT_M.3
PORT_M.2
Ro
3
OUT2
SNS1
PORT_M.4
Ro
4
FILT
POL
3.4 ESD PROTECTION
In some installations the QT114 will be protected from direct
static discharge by the insulation of the electrode and the
GATE OR
MICRO INPUT
2
OUT1
SNS2
CMOS
3
OUT2
SNS1
FILT
POL
6
100pF
Co
4
OUT1
SNS2
OUT2
SNS1
7
6
R2
4
POL
FILT
5
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.
Re2 functions to isolate the transient from the QT110's Vcc
pin; values of around 1K ohms are reasonable.
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.
If the QT114 is connected to an external circuit via a long
cable, it is possible for ground-bounce to cause damage to
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.
7
Co
100pF
3
5
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
timeconstant 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 non-critical capacitor from each used
OUT line to ground (Figure 3-4).
CMOS
2
R1
5
3.5 SAMPLE CAPACITOR
Figure 3-4 Eliminating HB Pulses
fact that the probe may not be accessible to human contact.
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.
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 QT114 does have diode protection on its terminals
which 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
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
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
-8-
Vcc
Re2
C1 10✙F
1
2 OUT1
SNS2 7
3 OUT2
SNS1 6
4 FILT
Re1
To Electrodes
CS
POL 5
8 Gnd
Figure 3-5 ESD Protection Network
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).
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.
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 CURRENT
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.
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.
Supply drain can be calculated from the adjusted voltage
droop using the basic charge equation:
i=
✁VC
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
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).
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.
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
timeconstants of settling with a 2µs charge time for efficient,
stable operation. 5% tolerance resistors can be used.
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.
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.
-9-
4.1 ABSOLUTE MAXIMUM SPECIFICATIONS
Operating temp . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . as designated by suffix
Storage temp . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . -55OC to +125OC
VCC . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . -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
VCC . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . +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
Parameter
Vcc = 3.0, Ta = recommended operating range
Description
Min
Typ
Max
TPC
Charge duration
2
TPT
Transfer duration
4
µs
TBS
Burst spacing interval
75
ms
0
µs
TBL
Burst length, time
NLC
Maximum burst length counts
500
3.5
counts
TR
Response time
80
ms
6
Notes
Units
14
ms
Slosh filter disabled
TQ
QT pulse spacing
TR
Pulse edge risetime
6
µs
ns
Cx = 5pF
TF
Pulse edge falltime
4
ns
Cx = 5pF
THB
Heartbeat pulse width
300
350
400
µs
Min
Typ
Max
Units
4.4 SIGNAL PROCESSING
Description
Notes
Threshold, T1
250
counts
Note 1
Threshold, T2
150
counts
Note 1
Hysteresis
3
counts
Note 1
Slosh filter length, time
15
seconds
Note 2
Slosh filter length, counts
190
counts
Note 1
Note 1: Counts of burst
Note 2: Uninterrupted detection / non-detection: Strap option.
- 10 -
4.5 DC SPECIFICATIONS
Vcc = 3.0V, Cs = 10nF, Cx = 10pF, TA = recommended range, unless otherwise noted
Parameter
Description
VCC
Supply voltage
IDD
Supply current
Min
Typ
Max
2.45
5.25
V
20
VDDS
Supply turn-on slope
VIL
Low input logic level
VHL
High input logic level
VOL
Low output voltage
VOH
High output voltage
IIL
Input leakage current
Notes
Units
µA
100
V/s
0.8
2.2
0.6
Vdd-0.8
±1
0
50
Required for proper startup
V
FILT, POL
V
FILT, POL
V
OUT1, OUT2, 5mA sink
V
OUT1, OUT2, 1mA source
µA
FILT, POL
CX
Load capacitance range
S[1]
Sensitivity [T1]
0.125
pF/count
pF
Cs = 20nF, Cx = 10pF
S[2]
Sensitivity [T2]
0.33
pF/count
Cs = 20nF, Cx = 30pF
FIGURE 4-1
FIGURE 4-2
Signal Level vs. Cx Load; Cs = 20nF; Vcc = 3.0
18
400
16
350
14
300
Signal, Counts
Counts per pF of Cx
Gain vs. Cx Load; Cs = 20nF; Vcc = 3.0
12
10
8
6
4
250
200
150
100
50
2
0
0.00
10.00
20.00
30.00
0
0.00
40.00
Cx Load, pF
10.00
20.00
30.00
40.00
Cx Load, pF
4.6 PACKAGING
AVAILABLE TYPES:
DIP-8
SO8N
0.26" / 6.5mm body, 0.100" pitch, plastic
0.15" / 3.9mm body, 0.050" pitch, plastic
D suffix
S suffix
Refer to QT110 datasheet for complete dimensional information
4.7 CUSTOMIZATION
QT114 technology can be customized to suit specific requirement, often with little NRE charge or change in part cost.
Consult your rep or the factory for further information, or email to: [email protected]
- 11 -
Quantum Research Group Ltd
©1999 QRG Ltd.
QProx, QTouch, QLevel, and HeartBeat are trademarks of QRG Ltd.
Patented and patents pending
651 Holiday Drive Bldg. 5 / 300
Pittsburgh, PA 15220 USA
Tel: 412-391-7367 Fax: 412-291-1015
[email protected]
http://www.qprox.com
In the United Kingdom
Enterprise House, Southampton, Hants SO14 3XB
Tel: +44 (0)23 8045 3934 Fax: +44 (0)23 8045 3939
Notice: This device expressly not for use in any medical or human safety related application
without the express written consent of an officer of the company.