REV. C

AD704

–9–

The 1 Hz, 4-pole active filter offers dc precision with a minimum

allow the use of 1 M

Ω 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

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 5. Table II 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

Filter Circuit

Table II. 1 Hz, 4-Pole Low-Pass Filter Recommended Component Values

Section 1

Section 2

Desired Low

Frequency

Frequency

C1

C2

C3

C4

Pass Response

(Hz)

Q

(Hz)

Q

( F)

( F)

( F)

( F)

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

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.