3.2 PIEZO SOUNDER
Figure 2-5 Eliminating HB Pulses
The use of a piezo sounder in place of Cs is described in the
previous section. Piezo sounders have very high,
uncharacterized thermal coefficients and should not be used
if fast temperature swings are anticipated, especially at high
gains.
GATE OR
MICRO INPUT
2
3
4
7
C M OS
OUT
SNS2
GAIN
SNS1
Co
100pF
5
6
3.3 OPTION STRAPPING
OPT1
OPT2
The option pins Opt1 and Opt2 should never be left floating.
If they are floated, the device will draw excess power and the
options will not be properly read on powerup. Intentionally,
there are no pullup resistors on these lines, since pullup
resistors add to power drain if tied low.
The Gain input is designed to be floated for sensing one of
the three gain settings. It should never be connected to a
pullup resistor or tied to anything other than Sns1 or Sns2.
metal disc will act as the sensing electrode. Piezo transducer
capacitances typically range from 6nF to 30nF (0.006µF to
0.03µF) in value; at the lower end of this range an additional
capacitor should be added to bring the total Cs across SNS1
and SNS2 to at least 10nF, or more if Cx is large.
Table 2-1 shows the option strap configurations available.
3.4 POWER SUPPLY, PCB LAYOUT
The power supply can range from 2.5 to 5.0 volts. At 3 volts
current drain averages less than 20µA in most cases, but
The burst acquisition process induces a small but audible
voltage step across the piezo resonator, which occurs when
SNS1 and SNS2 rapidly discharge residual voltage stored on
the resonator. The resulting slight clicking sound can be
used to provide an audible confirmation of functionality if
desired, or, it can be suppressed by placing a non-critical 1M
to 2M ohm bleed resistor in parallel with the resonator. The
resistor acts to slowly discharge the resonator, preempting
the occurrence of the harmonic-rich step (Figure 2-6).
Figure 2-6 Damping Piezo Clicks with R
x
+2.5 to 5
SENSING
ELECTRODE
With the resistor in place, an almost inaudible clicking sound
may still be heard, which is caused by the small charge
buildup across the piezo device during each burst.
1
Vdd
2
3
4
7
5
6
OUT
SNS1
GAIN
SNS2
2.2.6 OUTPUT
The QT118H’s `output is active high and it can source 1mA
DRIVE
OPT1
OPT2
or sink 5mA of non-inductive current.
R
C
x
x
Care should be taken when the IC and the load are both
powered from the same supply, and the supply is minimally
regulated. The device derives its internal references from the
power supply, and sensitivity shifts can occur with changes
in Vdd, as happens when loads are switched on. This can
induce detection ‘cycling’, whereby an object is detected, the
load is turned on, the supply sags, the detection is no longer
sensed, 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.
Vss
8
can be higher if Cs is large. Large Cx values will actually
decrease power drain. Operation can be from batteries, but
be cautious about loads causing supply droop (see Output
Drive, previous section).
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.
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.
3 - CIRCUIT GUIDELINES
3.1 SAMPLE CAPACITOR
When used for most applications, the charge sampler Cs
can be virtually any plastic film or ceramic capacitor. The
type should be relatively stable in the anticipated
temperature range. If fast temperature swings are expected,
especially with higher sensitivities, more stable capacitors be
required, for example PPS film, X7R, or NPO/C0G ceramic.
if desired, the supply can be regulated using a conventional
low current regulator, for example CMOS 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 QT118H's low
current requirement.
lq
6
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