REV. C

AD8011

–11–

1E+03

1E+04

1E+05

1E+06

1E+07

1E+08

1E+09

FREQUENCY (Hz)

140

120

100

80

60

20

0

40

0

–40

–80

–120

–160

–200

–240

–280

PHASE

GAIN

Figure 9. Open-Loop Transimpedance Gain

Note that the ac open-loop plots in Figures 8, 9, and 10 are based

on the full SPICE AD8011 simulations and do not include

external parasitics (see equations below). Nevertheless, these ac

loop equations still provide a good approximation to simulated

and actual performance up to the CLBW of the amplifier. Typi-

When the loop is closed however, the BW is greatly extended and

for s>>1/

τ1 (see Figure 10). This can be seen by the Z

Zs

g

mc

R

S

g

mc

R

g

mf

S

( –

)

–

()

=

×

×

+

















×+

11

1

11

1

21 1

τ

τ

As

G

G

A

R

T

S

G

g

mf

T

R

T

V

O

F

OO

F

O

()

=

++













+











 +

















11

2

1

τ

1E+03

1E+04

1E+05

1E+06

1E+07

1E+08

1E+09

400

370

340

310

280

220

190

FREQUENCY (Hz)

250

20

0

–20

–40

–120

160

130

100

–140

–160

–180

–60

–80

–100

SERIES 1

IMPEDANCE

SERIES 2

PHASE

Figure 10. Open-Loop Inverting Input Impedance

approaches

τ1. This results in the input resistance for the A

and the nominal design values for the other parameters, results in

a closed-loop 3 dB BW equal to the open-loop corner frequency

(1/2

1.2 M

Ω), the computed BW is 80 MHz for G = 0 (inverting

DRIVING CAPACITIVE LOADS

The AD8011 was designed primarily to drive nonreactive loads.

If driving loads with a capacitive component is desired, the best

settling response is obtained by the addition of a small series

resistance as shown in Figure 11. The accompanying graph shows

noting that the frequency response of the circuit when driving

large capacitive loads will be dominated by the passive roll-off

1k

1k

1k

AD8011

Figure 11. Driving Capacitive Load