AD810

REV. A

–12–

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 45 to chose the resistor

value, less than 1 dB of peaking can easily be achieved without

sacrificing much bandwidth. Note that the curves were

generated for the case of a 10 k

Ω load resistor, for smaller load

resistances, the peaking will be less than indicated by Figure 45.

Another method of compensating for large load capacitances is

to insert a resistor in series with the loop output as shown in

Figure 43. In most cases, less than 50

Ω is all that is needed to

achieve an extremely flat gain response.

Figures 44 to 46 illustrate the outstanding performance that can

be achieved when driving a 1000 pF capacitor.

AD810

7

3

2

3

0.1µF

6

4

0.1µF

1.0µF

1.0µF

Figure 43. Circuit Options for Driving a Large

Capacitive Load

0

1

10

100

–3

–6

–9

3

6

9

G = +2

FREQUENCY – MHz

Figure 44. Performance Comparison of Two Methods for

Driving a Large Capacitive Load

2k

1k

4k

3k

1

10

100

1000

FEEDBACK RESISTOR –

Ω

0

GAIN = +2

Figure 45. Max Load Capacitance for Less than 1 dB of

Peaking vs. Feedback Resistor

90

100

0%

5V

5V

100nS

V

IN

Figure 46. AD810 Driving a 1000 pF Load,

Ω, R

Ω, R

Ω

DISABLE MODE

By pulling the voltage on Pin 8 to common (0 V), the AD810

can be put into a disabled state. In this condition, the supply

current drops to less than 2.8 mA, the output becomes a high

impedance, and there is a high level of isolation from input to

output. In the case of a line driver for example, the output

impedance will be about the same as for a 1.5 k

Ω resistor (the

feedback plus gain resistors) in parallel with a 13 pF capacitor

(due to the output) and the input to output isolation will be

better than 65 dB at 1 MHz.

Leaving the disable pin disconnected (floating) will leave the

AD810 operational in the enabled state.

In cases where the amplifier is driving a high impedance load,

the input to output isolation will decrease significantly if the

input signal is greater than about 1.2 V peak to peak. The

isolation can be restored back to the 65 dB level by adding a

dummy load (say 150

Ω) at the amplifier output. This will

attenuate the feedthrough signal. (This is not an issue for

multiplexer applications where the outputs of multiple AD810s

are tied together as long as at least one channel is in the ON

state.) The input impedance of the disable pin is about 35 k

Ω in

parallel with a few pF. When grounded, about 50

µA flows out