AD8021 datasheet(18/20 Pages) AD

18 / 20 page

REV. D

–18–

AD8021

Figure 10 illustrates an inverter-follower driver circuit operating

at a gain of 2, using individually compensated AD8021s. The

values of feedback and load resistors were selected to provide a

total load of less than 1 k

Ω, and the equivalent resistances seen

at each op amp’s inputs were matched to minimize offset volt-

age and drift. Figure 12 is a plot of the resulting ac responses of

driver halves.

AD8021

–

7pF

249

499

G = +2

499

49.9

VOUT1

–VS

AD8021

–

5pF

232

G = –2

664

VOUT2

–VS

332

VIN

Figure 10. Differential Amplifier

G = –2

G = +2

FREQUENCY – Hz

100k

10M

100M

–3

–6

–9

–12

–15

–18

Figure 11. AC Response of Two Identically

Compensated High Speed Op Amps Configured

for Gains of +2 and –2

G =

100k

10M

100M

FREQUENCY – Hz

–3

–6

–9

–12

–15

–18

Figure12. AC Response of Two Dissimilarly

Compensated AD8021 Op Amps (Figure 11) Configured

for Gains of +2 and –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 litera-

ture 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 13 shows the schematic of a 2-pole, low-pass active filter,

and Table IV lists typical component values for filters having a

Bessel-type response with gains of 2 and 5. Figure 14 is a network

analyzer plot of this filter’s performance.

AD8021

VOUT

+VS

VIN

–VS

Figure 13. Schematic of a Second-Order Low-Pass

Active Filter

Table IV. Typical Component Values for Second-Order

Low-Pass Filter of Figure 13

Gain

R1 ( )R2 ( )RF ( )RS ( )C1

C2CC

271.5

215

499

10 nF

7 pF

544.2

365

90.9

365

10 nF

2 pF

10k

100k

10M

FREQUENCY – Hz

–10

–20

–30

–40

–50

G = 2

G = 5

Figure 14. Frequency Response of the Filter Circuit

of Figure 13 for Two Different Gains

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