
AD705
REV. B
–8–
P
C
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
–V
S
7
0.1μF
+V
S
AD705
V
OUT
C1
C3
0.01μF
R3
2M
R2
1M
C2
R1
1M
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
Pass Response
Pole
Frequency
(Hz)
Pole Q
C1 Value
C2 Value
(
m
F)
(
m
F)
Bessel Response
Butterworth Response
0.1 dB Chebychev
0.2 dB Chebychev
0.5 dB Chebychev
1.0 dB Chebychev
1.27
1.00
0.93
0.90
0.85
0.58
0.707
0.77
0.80
0.86
0.96
0.14
0.23
0.26
0.28
0.32
0.38
0.11
0.11
0.11
0.11
0.11
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
μ
F, C2 = 0.037
μ
F.
90
–90
+140
0
–60
–40
–30
–60
60
30
+120
+80
+60
+40
+100
+20
0
–20
O
TEMPERATURE –
°
C
WITHOUT OPTIONAL
BALANCE RESISTOR, R3
WITH OPTIONAL BALANCE
RESISTOR, R3
Figure 27. V
OS
vs. Temperature of 1 Hz Filter
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
0-15
°
0.32 (8.13)
0.29 (7.37)
PIN 1
4
5
1
8
0.0500
(1.27)
BSC
(3.91
±
±
0.004
(6.00
±
±
0.012
(4.90
±
±
0.008
±
0.098
±
0.006
±
(0.008
±
0.004
±
0.017
±
0.003
±
0.03)
0.011
±
±
0.033
±
0.017
PIN 1
(0.25
4
5
8
1
SEATING
PLANE
0.100
(2.54)
TYP
(0.31
0.39 (9.91)
MAX
±
0.035
±
0.01
±
0.76)
0.18
±
0.033
(0.84)
NOM
(0.46
±
±
0.003
0.125 (3.18)
MIN
±
0.165
±
0.01
0.30 (7.62)
REF
(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