HT761X

Rev. 1.30

6

October 12, 2009

1

16

,

1

11.3

or

1

9

The default is

1

16

PIR Amplifier

Consult the diagram below for details on the PIR front

end amplifier.

In the Fig.2 below there are 2 op-amps with different ap-

plications. OP1 can be used independently as a first

stage inverting or non-inverting amplifier for the PIR.

As the output of OP2 is directly connected to the input of

the comparator, it is used as a second stage amplifying

device. The non-inverting input of OP2 is connected to

the comparator

¢s window center point and can be used

to check this voltage and to provide a bias voltage that is

equal to the center point voltage of the comparator. In

Fig.2 the comparator can have 3 window levels set by

mask option. 1.

1

16

1

11.3

3.

1

9

If the window level fails to be specified the default win-

dow is set to

1

16

therefore 0.25V, ()

4

16

V .

Second Stage Amplifier

Usually the second stage PIR amplifier is a simple ca-

pacitively 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.3 OP2P is directly connected to the comparator

window center, and with the C3 filter it can act as the

R2

R1

, low cutoff

1

2R1C1

, high cutoff frequency

1

R2C2

2

. By changing the value of R2 the sensitiv-

ity can be varied. C1 and C3 should be of low leakage

types to prevent the DC operating point from changing

due to current leakage.

Each op-amp current consumption is approximately

5

mA with the op-amps and comparator¢s working volt-

age all provided by the regulator.

Consult the following diagrams for typical PIR front end

circuit.

First Stage of PIR Amplifier

Fig.4 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.

(R1+ R2 )

R1

O P 1

O P 2

R e g u l a t o r

C o m p a r a t o r

O P 1 O

O P 1 N

O P 1 P

O P 2 O

O P 2 N

O P 2 P

V E E

Fig.2 PIR Amplifier

R 1

2 2 k W

f i r s t s t a g e

o u t p u t

C 1

2 2 m F

C 2

0 . 0 2 2 m F

O P 2 N

O P 2 P

O P 2

O P 2 O

R W

R W

V E E

R W

R W

C 3

1 0 m F

R 2

1 M W

Fig.3 Typical Second Stage Amplifier

p

p