OP176GS datasheet(15/21 Pages) AD

15 / 21 page

OP176

REV. 0

–15–

High Pass Sections

Figure 44a illustrates the high-pass form of a 2-pole SK filter

using an OP176. For simplicity and practicality, capacitors C1

and C2 are set equal (“C”), and resistors R2 and R1 are

adjusted to a ratio, N, which provides the filter damping

coefficient,

α, as per the design expressions. This high pass

design is begun with selection of standard capacitor values for

C1 and C2 and a calculation of N. The values for R1 and R2

are then determined from the following expressions:

π× FREQ × C × N

and

= N × R1

Figures 44a. Two-Pole Unity Gain HP/LP Active Filters

In this examples, circuit

α (or 1/Q) is set equal to √2, providing

a Butterworth (maximally flat) characteristic. The filter corner

frequency is normalized to 1 kHz, with resistor values shown in

both rounded and (exact) form. Various other 2-pole response

shapes are possible with appropriate selection of

α, and fre-

quency can be easily scaled, using inversely proportional R or

C values for a given

α. The 22 V/µs slew rate of the OP176 will

support 20 V p-p outputs above 100 kHz with low distortion.

The frequency response resulting with this filter is shown as the

dotted HP portion of Figure 45.

+VS

11k

(11.254k)

0.01µF

OUT

OP176

0.01µF

–VS

22k

(22.508k)

GIVEN:

α, FREQ

SET C1 = C2 = C

α =

N =

α2

R1 =

R2 = N x R1

ZCOMP

ZCOMP (HIGH PASS)

IN (–)

OUTPUT

1 kHz BW SHOWN

π FREQ x C x N

Low Pass Sections

In the LP SK arrangement of Figure 44b, the R and C elements

are interchanged where the resistors are made equal. Here, the

ratio of C2/C1 (“M”) is used to set the filter

α, as noted.

Otherwise, this filter is similar to the HP section, and the

resulting 1 kHz LP response is shown in Figure 45. The design

begins with a choice of a standard capacitor value for C1 and a

calculation of M. This then forces a value of “M

× C1” for C2.

Then, the value for R1 and R2 (“R”) is calculated according to

the following equation:

π× FREQ × C1× M

Figures 44b. Two-Pole Unity Gain HP/LP Active Filters

Figure 45. Relative Frequency Response of 2-Pole, 1 kHz

Butterworth LP (Left) and HP (Right) Active Filters

+VS

11k

(11.254k)

0.02µF

OUT

OP176

0.01µF

–VS

11k

(11.254k)

GIVEN:

α, FREQ

α =

M =

α2

C2 = M x C1

ZCOMP

IN (–)

OUTPUT

1 kHz BW SHOWN

CHOOSE C1

R =

π FREQ x C1 x M

ZCOMP (LOW PASS)

100

50k

10k

10.000

–30.00

–70.00

–50.00

–10.00

–20.00

–40.00

–60.00

0.0

FREQUENCY – Hz

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