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,

IOS, and IB 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 IB is as low as its IOS, 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. Table 5 gives

capacitor values for several common low pass responses.

180

120

60

0

–60

–120

–180

–40

0

40

80

120

TEMPERATURE (°C)

WITHOUT OPTIONAL

BALANCE RESISTOR, R3

WITH OPTIONAL

BALANCE RESISTOR, R3

Figure 36. VOS vs. Temperature Performance of the 1 Hz Filter Circuit

Table 5. 1 Hz, Four-Pole Low-Pass Filter Recommended Component Values1

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.