AD705

REV. B

–8–

A 1 Hz, 2-Pole, Active Filter

Table II gives recommended component values for the 1 Hz fil-

ter of Figure 26. An unusual characteristic of the AD705 is that

both the input bias current and the input offset current and their

drift remain low over most of the op amps rated temperature

range. Therefore, for most applications, there is no need to use

the normal balancing resistor tied between the noninverting ter-

minal of the op amp and ground. Eliminating the standard bal-

ancing resistor reduces board space and lowers circuit noise.

However, this resistor is needed at temperatures above 110

°C,

because input bias current starts to change rapidly, as shown by

Figure 27.

6

4

0.1µF

7

0.1µF

AD705

C1

C3

0.01µF

R3

2M

Ω

R2

1M

Ω

C2

R1

1M

Ω

INPUT

OPTIONAL BALANCE

RESISTOR NETWORK

WITHOUT THE NETWORK,

PINS 2 AND 6 OF THE AD705

ARE TIED TOGETHER.

CAPACITORS C1, C2 AND C3 ARE SOUTHERN ELECTRONICS MPCC,

POLYCARBONATE,

±5%, 50 VOLT.

2

3

Figure 26. A 1 Hz, 2-Pole Active Filter

Table II. Recommended Component Values

for the 1 Hz Low-Pass Filter

Desired Low

Pole

Pole Q

C1 Value

C2 Value

Pass Response

Frequency

(Hz)

( F)

( F)

Bessel Response

1.27

0.58

0.14

0.11

Butterworth Response

1.00

0.707

0.23

0.11

0.1 dB Chebychev

0.93

0.77

0.26

0.11

0.2 dB Chebychev

0.90

0.80

0.28

0.11

0.5 dB Chebychev

0.85

0.86

0.32

0.11

1.0 dB Chebychev

0.80

0.96

0.38

0.10

Specified values are for a –3 dB point of 1.0 Hz. For other frequencies,

simply scale capacitors C1 and C2 directly; i.e., for 3 Hz Bessel response,

C1 = 0.046

µF, C2 = 0.037 µF.

90

–90

+140

0

–60

–40

–30

–60

60

30

+120

+80

+60

+40

+100

+20

0

–20

TEMPERATURE –

°C

WITHOUT OPTIONAL

BALANCE RESISTOR, R3

WITH OPTIONAL BALANCE

RESISTOR, R3

0.310 (7.87)

0.220 (5.59)

0.015 (0.38)

0.008 (0.20)

0.005 (0.13) MIN

0.055 (1.4) MAX

1

4

5

8

0.405 (10.29) MAX

0.200

(5.08)

MAX

SEATING

PLANE

0.023 (0.58)

0.014 (0.36)

0.070 (1.78)

0.030 (0.76)

0.060 (1.52)

0.015 (0.38)

0.150

(3.81)

MIN

0.200 (5.08)

0.125 (3.18)

0.100

(2.54)

BSC

0.25R

(0.64)

0-15

°

0.32 (8.13)

0.29 (7.37)

PIN 1

4

5

1

8

0.0500

(1.27)

BSC

0.154

± 0.004

(3.91

± 0.10)

0.236

± 0.012

(6.00

± 0.20)

0.193

± 0.008

(4.90

± 0.10)

0.098

± 0.006

(2.49

± 0.23)

0.008

± 0.004

(0.203

± 0.075)

0.017

± 0.003

(0.42

± 0.07)

0.011

± 0.002

(0.269

± 0.03)

0.033

± 0.017

(0.83

± 0.43)

PIN 1

0.25

(6.35)

4

5

8

1

SEATING

PLANE

0.100

(2.54)

TYP

0.31

(7.87)

0.39 (9.91)

MAX

0.035

± 0.01

(0.89

± 0.25)

0.18

± 0.03

(4.57

± 0.76)

0.033

(0.84)

NOM

0.018

± 0.003

(0.46

± 0.08)

0.125 (3.18)

MIN

0.165

± 0.01

(4.19

± 0.25)

0.30 (7.62)

REF

0.011

± 0.003

(0.28

± 0.08)

0-15

°

OUTLINE DIMENSIONS

Dimensions shown in inches and (mm).

Cerdip (Q) Package

Plastic Mini-DIP (N) Package

8-Pin SOIC (R) Package