AD795JRZ datasheet(14/20 Pages) AD

14 / 20 page

AD795

Rev. C | Page 14 of 20

AC RESPONSE WITH HIGH VALUE SOURCE AND FEEDBACK RESISTANCE

Source and feedback resistances greater than 100 kΩ magnifies

the effect of input capacitances (stray and inherent to the

AD795) on the ac behavior of the circuit. The effects of

common-mode and differential input capacitances should be

taken into account because the circuit’s bandwidth and stability

can be adversely affected.

In a follower, the source resistance, RS, and input common-

mode capacitance, CS (including capacitance due to board and

capacitance inherent to the AD795), form a pole that limits

circuit bandwidth to 1/2 π RSCS. Figure 38 shows the follower

pulse response from a 1 MΩ source resistance with the

amplifier’s input pin isolated from the board; only the effect of

the AD795’s input common-mode capacitance is seen.

100

10mV

5µs

Figure 38. Follower Pulse Response from 1 MΩ Source Resistance

In an inverting configuration, the differential input capacitance

forms a pole in the circuit’s loop transmission. This can create

peaking in the ac response and possible instability. A feedback

capacitance can be used to stabilize the circuit. The inverter

pulse response with RF and RS equal to 1 MΩ and the input pin

isolated from the board appears in Figure 39. Figure 40 shows

the response of the same circuit with a 1 pF feedback

capacitance. Typical differential input capacitance for the

AD795 is 2 pF.

100

10mV

5µs

Figure 39. Inverter Pulse Response with 1 MΩ Source and Feedback

Resistance

100

10mV

5µs

Figure 40. Inverter Pulse Response with 1 MΩ Source and Feedback

Resistance, 1 pF Feedback Capacitance

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