HT761X Datasheet, PDF (10/12 Pages) Holtek Semiconductor Inc – General Purpose PIR Controller

Second stage amplifier
HT761X
Fig.6 Typical second stage amplifier
Usually the second stage PIR amplifier is a
simple capacitively coupled inverting amplifier
with a low pass configuration. The noninverting
input terminal is biased to the center point of
the comparator window and the output of the
second stage amplifier is directly coupled to the
comparator center point.
In Fig.6 OP2P is directly connected to the com-
parator window center, and with the C3 filter it
can act as the bias for OP2. For this configura-
tion
AV
=
RR21,
low
cutoff
frequency
fL
=
1
2πR1C1
,
high
cutoff
frequency
fH
=
1
2πR2C2
.
By
chang-
ing the value of R2 the sensitivity can be varied.
C1 and C3 should be of low leakage types to
prevent the DC operating point from change
due to current leakage.
Each op-amp current consumption is approxi-
mately 5µA with the op-amps and comparator’s
working voltage all provided by the regulator.
Consult the following diagrams for typical PIR
front end circuit.
Fig.7 and Fig.8 are similar but in Fig.8 the
input signal of amplifier is taken from the drain
of the PIR. This has higher gain than that in
Fig.7. Since OP1 is a PMOS input VD has to be
greater than 1.2V for adequate operation.
Fig. 7 Typical first-stage PIR
First stage of PIR amplifier
Fig.7 shows a typical first stage amplifier. C2
and R2 form a simple low pass filter with cut off
frequency at 7Hz. The low frequency response
is governed by R1 and C1 with cut-off frequency
at 0.33Hz.
AV=
(
R1+ R2
R1
)
Fig.8 High gain first stage
10
24th Mar ’97