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OP176

REV. 0

–16–

Passive Component Selection for Active Filters

The passive components suitable for active filters deserve more

than casual attention. Resistors should be 1%, low TC, metal-

film types of the RN55 or RN60 style. Capacitors should be 1%

or 2% film types preferably, such as polypropylene or polysty-

rene, or NPO (COG) ceramic for smaller values.

Active Filter Circuit Subtleties

In designing active filter circuits with the OP176, moderately

low values (10 k

Ω or less) for R1 and R2 can be used to

minimize the effects of Johnson noise when critical. The

practical tradeoff is, of course, capacitor size and expense. DC

errors will result for larger values of resistance, unless compen-

sation for amplifier input bias current is used. To add bias

compensation in the HP filter section of Figure 42a, a feedback

compensation resistor equal to R2 can be used. This will

minimize bias current induced offset to the product of the

OP176’s I

OS and R2. For an R2 of 25 kΩ, this produces a typical

compensated offset voltage of 50

µV. Similar compensation is

applied to Figure 42b, using a resistance equal to R1+ R2.

Using dc compensation, filter output dc errors using the OP176

will be dominated by its V

OS, which is typically 1 mV or less.

caveat here is that the additional resistors can increase noise

substantially. For example, a 10 k

Ω resistor generates ~ 12 nV/

√Hz of noise and is about twice that of the OP176. These

resistors can be ac bypassed to eliminate their noise using a

simple shunt capacitor chosen such that its reactance (X

C) is

much less than R at the lowest frequency of interest.

A more subtle form of ac degradation is also possible in these

filters, namely nonlinear input capacitance modulation. This

issue was previously covered for general cases in the section on

minimizing distortion. In active filter circuits, a fully compen-

sating network (for both dc and ac performance) can be used to

minimize this distortion. To be most effective, this network

COMP) should include R1 through C2 as noted for either filter

type, of the same style and value as their counterparts in the

forward path. The effects of a Z

COMP network on the THD + N

performance of two 1 kHz HP filters is illustrated in Figure 46.

One filter (A) is the example shown in Figure 44a (Curves A1

and A2), while the second (B) uses RC values scaled 10 times

upward in impedance (Curves B1 and B2). Both filters operate

with a 2 V rms input,

±18 V supplies, 100 kΩ loading, and

analyzer bandwidth of 80 kHz.

Figure 46. THD + N (%) vs. Frequency for Various 1 kHz HP

Active Filters Illustrating the Effects of the Z

COMP Network

Curves A1 and B1 show performance with Z

COMP shorted,

while curves A2 and B2 illustrate operation with Z

COMP active.

For the “A” example values, distortion in the pass band of

1 kHz–20 kHz is below 0.001% compensated, and slightly

higher uncompensated. With the higher impedance “B” net-

work, there is a much greater difference between compensated

and uncompensated responses, underscoring the sensitivity to

higher impedances. Although the positive effect of Z

COMP is seen

for both “A” and “B” cases, there is a buffering effect which

takes place with lower impedances. As case “A” shows, when

using larger capacitance values in the source, the amplifier’s

nonlinear C-V input characteristics have less effect on the

signal.

Thus, to minimize the necessity for the complete Z

COMP com-

pensation, effective filter designs should use the lowest capaci-

tive impedances practical, with an 0.01

µF lower value limit as a

goal for lowest distortion (while lower values can certainly be

used, they may suffer higher distortion without the use of full

compensation). Since most designs are likely to use low relative

impedances for reasons of low noise and offset, the effects of

CM distortion may or may not actually be apparent to a given

application.

100

20k

0.010

0.0001

0.001

0.1

10 k

FREQUENCY – Hz

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