AD8021 datasheet(15/20 Pages) AD

15 / 20 page

REV. D

AD8021

–15–

Table I. Recommended Component Values. See Test Circuit 2. CF = CL = 0, RL = 1 k , RIN = 49.9

Noise Gain

Slew

–3 dB

Output Noise

(Noninverting

CCOMP

Rate

SS BW

(AD8021 Only)

(AD8021 with Resistors)

Gain)

( )( )( )

(pF)

(V/ s)

(MHz)

(nV/

√Hz)

(nV/

√Hz)

120

490

2.1

2.8

49.9

499

150

205

4.3

8.2

49.9

1 k

249

300

185

10.7

15.5

49.9

1 k

110

420

150

21.2

27.9

49.9

1 k

52.3

200

42.2

52.7

100

49.9

1 k

211.1

264.1

APPLICATIONS

The typical voltage feedback op amp is frequency stabilized with

a fixed internal capacitor, CINTERNAL, using dominant pole compen-

sation. To a first-order approximation, voltage feedback op amps

have a fixed gain bandwidth product. For example, if its –3 dB

bandwidth for G = +1 is 200 MHz, at a gain of G = +10 its

bandwidth will be only about 20 MHz. The AD8021 is a voltage

feedback op amp with a minimal CINTERNAL of about 1.5 pF. By

adding an external compensation capacitor, CC, the user can

circumvent the fixed gain bandwidth limitation of other voltage

feedback op amps.

Unlike the typical op amp with fixed compensation, the AD8021

allows the user to

Maximize the amplifier bandwidth for closed-loop gains

between 1 and 10, avoiding the usual loss of bandwidth

and slew rate.

Optimize the trade-off between bandwidth and phase

margin for a particular application.

Match bandwidth in gain blocks with different noise gains,

such as when designing differential amplifiers (as shown in

Figure 10).

FREQUENCY – Hz

100M

110

10k

10M

100

100k

CC = 10pF

10G

–10

180

135

CC = 0pF (B)

(C)

(A)

(B)

(C)

Figure 3. Simplified Diagram of Open-Loop Gain

and Phase Response

Figure 3 is the AD8021 gain and phase plot that has been sim-

plified for instructional purposes. If the desired closed-loop gain

is G = +1 and CC = 10 pF is chosen, Arrow A of the figure

shows that the bandwidth is about 200 MHz and the phase

margin is about 60

°. If the gain is changed to G = +10 and CC

is fixed at 10 pF, then (as expected for a typical op amp) the

bandwidth is degraded to about 20 MHz and the phase margin

increases to 90

° (Arrow B). However, by reducing C

C to zero,

the bandwidth and phase margin return to about 200 MHz and

° (Arrow C), respectively. In addition, the slew rate is dra-

matically increased, as it roughly varies with the inverse of CC.

123456789

NOISE GAIN – V/V

Figure 4. Suggested Compensation Capacitance

vs. Gain for Maintaining 1 dB Peaking

Table I and Figure 4 provide recommended values of compensa-

tion capacitance at various gains and the corresponding slew rate,

bandwidth, and noise. Note that the value of the compensation

capacitor depends on the circuit noise gain, not the voltage gain.

As shown in Figure 5, the noise gain, GN, of an op amp gain block

is equal to its noninverting voltage gain, regardless of whether

it is actually used for inverting or noninverting gain. Thus,

Noninverting G

Inverting G

AD8021

CCOMP

–VS

800

200

–

G = GN = 5

AD8021

CCOMP

–VS

–

G = –4

GN = 5

800

200

NONINVERTING

INVERTING

Figure 5. The Noise Gain of Both Is 5

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