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AD8021

Rev. F | Page 23 of 28

G = –2

G = +2

FREQUENCY (Hz)

100k

10M

100M

–3

–6

–9

–12

–15

–18

Figure 66. AC Response of Two Identically Compensated High Speed Op

Amps Configured for a Gain of +2 and a Gain of −2

100k

10M

100M

FREQUENCY (Hz)

–3

–6

–9

–12

–15

–18

G = ±2

Figure 67. AC Response of Two Dissimilarly Compensated AD8021 Op Amps

(Figure 66) Configured for a Gain of +2 and a Gain of −2,

(Note the Close Gain Match)

USING THE AD8021 IN ACTIVE FILTERS

The low noise and high gain bandwidth of the AD8021 make it

an excellent choice in active filter circuits. Most active filter

literature provides resistor and capacitor values for various

filters but neglects the effect of the op amp’s finite bandwidth on

filter performance; ideal filter response with infinite loop gain is

implied. Unfortunately, real filters do not behave in this manner.

Instead, they exhibit finite limits of attenuation, depending on

the gain bandwidth of the active device. Good low-pass filter

performance requires an op amp with high gain bandwidth for

attenuation at high frequencies, and low noise and high dc gain

for low frequency, pass-band performance.

Figure 68 shows the schematic of a 2-pole, low-pass active filter

and lists typical component values for filters having a Bessel-

type response with a gain of 2 and a gain of 5. Figure 69 is a

network analyzer plot of this filter’s performance.

AD8021

VOUT

+VS

VIN

–VS

Figure 68. Schematic of a Second-Order, Low-Pass Active Filter

Table 9. Typical Component Values for Second-Order, Low-

Pass Active Filter of Figure 68

Gain

(Ω)

(nF)

(pF)

71.5

215

499

44.2

365

90.9

10k

100k

10M

FREQUENCY (Hz)

–10

–20

–30

–40

–50

G = 2

G = 5

Figure 69. Frequency Response of the Filter Circuit of Figure 68

for Two Different Gains

DRIVING CAPACITIVE LOADS

When the AD8021 drives a capacitive load, the high frequency

response can show excessive peaking before it rolls off. Two

techniques can be used to improve stability at high frequency

and reduce peaking. The first technique is to increase the

compensation capacitor, CC, which reduces the peaking while

maintaining gain flatness at low frequencies. The second

technique is to add a resistor, RSNUB, in series between the output

pin of the AD8021 and the capacitive load, C

shows

the response of the AD8021 when both C

Figure 70

and RSNUB

are used to

reduce peaking. For a given CL, Figure 71 can be used to

determine the value of RSNUB that maintains 2 dB of peaking in

the frequency response. Note, however, that using R

SNUB

attenuates

the low frequency output by a factor of RLOAD/(RSNUB

+ RLOAD).

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