AD810 datasheet(17/22 Pages) AD

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Data Sheet

AD810

Rev. B | Page 17 of 22

APPLICATIONS INFORMATION

CAPACITIVE LOADS

When used with the appropriate feedback resistor, the AD810

can drive capacitive loads exceeding 1000 pF directly without

oscillation. By using the curves in Figure 50 to choose the

resistor value, less than 1 dB of peaking can easily be achieved

without sacrificing much bandwidth. Note that the curves in

Figure 50 were generated for the case of a 10 kΩ load resistor.

For smaller load resistances, the peaking is less than indicated

by Figure 50.

Another method of compensating for large load capacitances is

to insert a resistor in series (RS) with the loop output, as shown

in Figure 48. In most cases, less than 50 Ω is all that is needed to

achieve an extremely flat gain response.

Figure 49 to Figure 51 illustrate the outstanding performance

that can be achieved when driving a 1000 pF capacitor.

RS (OPTIONAL)

AD810

–VS

+VS

0.1µF

1µF

0.1µF

1µF

VIN

VOUT

Figure 48. Circuit Options for Driving a Large Capacitive Load

–15

–12

–9

–6

–3

100

FREQUENCY (MHz)

GAIN = +2

VS = ±15V

RL = 10kΩ

CL = 1000pF

RF = 4.5kΩ

RS = 0Ω

RF = 750Ω

RS = 11Ω

Figure 49. Performance Comparison of Two Methods for Driving a Large

Capacitive Load

10k

100

FEEDBACK RESISTOR (Ω)

GAIN = +2

RL = 10kΩ

VS = ±5V

VS = ±15V

Figure 50. Maximum Load Capacitance vs. Feedback Resistor,

Peaking < 1 dB

100

100ns

VIN

VOUT

Figure 51. AD810 Driving a 1000 pF Load, Gain = +2, RF = 750 Ω, RS = 11 Ω,

RL = 10 kΩ

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