AD704
The 1 Hz, 4-pole active filter offers dc precision with a mini-
mum of components and cost. The low current noise, I
OS
, and
I
B
allow the use of 1 MΩ resistors without sacrificing the
1
µV/°C
drift of the AD704. This means lower capacitor values
may be used, reducing cost and space. Furthermore, since the
AD704’s I
B
is as low as its I
OS
, over most of the MIL tempera-
ture range, most applications do not require the use of the
normal balancing resistor (with its stability capacitor). Adding the
optional balancing resistor enhances performance at high
temperatures, as shown in Figure 26. Table II gives capacitor
values for several common low pass responses.
180
120
OFFSET VOLTAGE
OF FILTER CIRCUIT (RTI) –
µV
WITHOUT OPTIONAL
BALANCE RESISTOR, R3
60
0
–60
–120
–180
–40
0
+40
+80
o
TEMPERATURE – C
+120
Figure 26. V
OS
vs. Temperature Performance of the 1 Hz
Filter Circuit
Table II. 1 Hz, 4-Pole Low-Pass Filter Recommended Component Values
Desired Low
Pass Response
Bessel
Butterworth
0.1 dB Chebychev
0.2 dB Chebychev
0.5 dB Chebychev
1.0 dB Chebychev
Section 1
Frequency
(Hz)
1.43
1.00
0.648
0.603
0.540
0.492
Q
0.522
0.541
0.619
0.646
0.705
0.785
Section 2
Frequency
(Hz)
1.60
1.00
0.948
0.941
0.932
0.925
Q
0.806
1.31
2.18
2.44
2.94
3.56
C1
( F)
0.116
0.172
0.304
0.341
0.416
0.508
C2
( F)
0.107
0.147
0.198
0.204
0.209
0.206
C3
( F)
0.160
0.416
0.733
0.823
1.00
1.23
C4
( F)
0.0616
0.0609
0.0385
0.0347
0.0290
0.0242
Specified Values are for a –3 dB point of 1.0 Hz. For other frequencies simply scale capacitors C1 through C4 directly; i.e., for 3 Hz Bessel response, C1 = 0.0387
µF,
C2 = 0.0357
µF,
C3 = 0.0533
µF,
C4 = 0.0205
µF.
OUTLINE DIMENSIONS
Dimensions shown in inches and (mm).
14-Pin Cerdip (Q) Package
14-Pin Plastic DIP (N) Package
16-Pin Plastic SO (R) Package
20-Terminal LCCC (E) Package
0.100 (2.54)
0.064 (1.63)
0.358 (9.09)
0.342 (8.69)
0.040 (1.02)
x 45° REF
3 PLCS
NO. 1 PIN
INDEX
0.050
(1.27)
BSC
0.028 (0.71)
0.022 (0.56)
0.020 (0.51)
x 45° REF
–8–
REV. A
PRINTED IN U.S.A.
C1476–24–10/90
WITH OPTIONAL
BALANCE RESISTOR, R3
AD [ ANALOG DEVICES ]
