Figure 2-4
Getting HB pulses with a pull-down resistor
Figure 2-5
Using a micro to obtain HB pulses in either output state
+2.5 to 5
HeartBeat™ Pulses
1
PORT_M.x
2
3
4
7
5
6
Vdd
2
3
4
7
5
6
OUT
SNS2
GAIN
SNS1
OUT
SNS2
GAIN
SNS1
R
o
Ro
Microprocessor
OPT1
OPT2
OPT1
OPT2
PORT_M.y
Vss
8
the load is turned off, the supply rises and the object is
reacquired, ad infinitum. To prevent this occurrence, the output
should only be lightly loaded if the device is operated from an
unregulated supply, e.g. batteries. Detection ‘stiction’, the
opposite effect, can occur if a load is shed when Out is active.
to reduce stray loading (which will dramatically reduce
sensitivity).
2. Keep Cs, Rs, and Re very close to the IC.
3. Make Re as large as possible. As a test, check to be sure
that an increase of Re by 50% does not appreciably
decrease sensitivity; if it does, reduce Re until the 50%
test increase has a negligible effect on sensitivity.
The output of the QT110 can directly drive a resistively limited
LED. The LED should be connected with its cathode to the
output and its anode towards Vcc, so that it lights when the
sensor is active-low. If desired the LED can be connected from
Out to ground, and driven on when the sensor is inactive, but
only with less drive current (1mA).
4. Do not route the sense wire near other ‘live’ traces
containing repetitive switching signals; the sense trace will
pick up noise from them.
3.3 POWER SUPPLY, PCB LAYOUT
See also Section 3.4.
3 - CIRCUIT GUIDELINES
3.1 SAMPLE CAPACITOR
The power supply can range from 2.5 to 5.0 volts. At 2.5 volts
current drain averages less than 10µA with Cs = 10nF,
provided a 470K Rs resistor is used (Figure 2-6). Idd curves
are shown in Figure 4-4.
When used for most applications, the charge sampler Cs can
be virtually any plastic film or good quality ceramic capacitor.
The type should be relatively stable in the anticipated
temperature range. If fast temperature swings are expected,
especially at higher sensitivity, a more stable capacitor might
be required for example PPS film.
Higher values of Cs will raise current drain. Higher Cx values
can actually decrease power drain. Operation can be from
batteries, but be cautious about loads causing supply droop
(see Output Drive, Section 2.2.6) if the batteries are
unregulated.
In most moderate applications a low-cost X7R type will work
fine.
As battery voltage sags with use or fluctuates slowly with
temperature, the IC will track and compensate for these
changes automatically with only minor changes in sensitivity.
3.2 ELECTRODE WIRING
See also Section 3.4.
The wiring of the electrode and its connecting trace is important
to achieving high signal levels and low noise. Certain design
rules should be adhered to for best results:
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
device. The IC will track slow changes in Vdd, but it can be
affected by rapid voltage steps.
1. Use a ground plane under the IC itself and Cs and Rs but
NOT under Re, or under or closely around the electrode or
its connecting trace. Keep ground away from these things
if desired, the supply can be regulated using a conventional
low current regulator, for example CMOS LDO regulators that
have nanoamp quiescent currents. Care should be taken that
the regulator does not have a minimum load specification,
which almost certainly will be violated by the QT110's low
current requirement. Furthermore, some LDO regulators are
unable to provide adequate transient regulation between the
quiescent and acquire states, creating Vdd disturbances that
will interfere with the acquisition process. This can usually be
solved by adding a small extra load from Vdd to ground, such
as 10K ohms, to provide a minimum load on the regulator.
Figure 2-6 Eliminating HB Pulses
GATE OR
MICRO INPUT
2
3
4
7
5
6
CMO S
OUT
SNS2
GAIN
SNS1
Co
100pF
OPT1
OPT2
Conventional non-LDO type regulators are usually more stable
than slow, low power CMOS LDO types. Consult the regulator
manufacturer for recommendations.
For proper operation a 100nF (0.1uF) ceramic bypass
capacitor must be used between Vdd and Vss; the bypass cap
LQ
6
QT110 R1.04/0405
QUANTUM [ QUANTUM RESEARCH GROUP ]
