AD704
Rev. E | Page 12 of 16
The 1 Hz, four-pole active filter offers dc precision with a
minimum 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 that lower capacitor
values can be used, reducing cost and space. Furthermore,
because the AD704’s I
B
is as low as its I
OS
, over most of the MIL
temperature 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 36. Tabl e 5 gives
capacitor values for several common low pass responses.
180
120
60
0
–60
–120
–180
–40 0 40 80 120
TEMPERATURE (°C)
OFFSET VOLTAGE
OF FILTER CIRCUIT (RTI) (µV)
WITHOUT OPTIONAL
BALANCE RESISTOR, R3
WITH OPTIONAL
BALANCE RESISTOR, R3
00818-036
Figure 36. V
OS
vs. Temperature Performance of the 1 Hz Filter Circuit
Table 5. 1 Hz, Four-Pole Low-Pass Filter Recommended Component Values
1
Desired Low Pass
Response
Section 1
Frequency (Hz)
Q
Section 2
Frequency (Hz)
Q C1 ( µF) C2 (µF) C3 (µF) C4 (µF)
Bessel 1.43 0.522 1.60 0.806 0.116 0.107 0.160 0.0616
Butterworth 1.00 0.541 1.00 1.31 0.172 0.147 0.416 0.0609
0.1 dB Chebychev 0.648 0.619 0.948 2.18 0.304 0.198 0.733 0.0385
0.2 dB Chebychev 0.603 0.646 0.941 2.44 0.341 0.204 0.823 0.0347
0.5 dB Chebychev 0.540 0.705 0.932 2.94 0.416 0.209 1.00 0.0290
1.0 dB Chebychev 0.492 0.785 0.925 3.56 0.508 0.206 1.23 0.0242
1
Specified values are for a −3 dB point of 1.0 Hz. For other frequencies, simply scale the C1 through C4 capacitors directly; that is, for a 3 Hz Bessel response,
C1 = 0.0387 μF, C2 = 0.0357 μF, C3 = 0.0533 μF, and C4 = 0.0205 μF.
