AD795

REV. A

–11–

AC RESPONSE WITH HIGH VALUE SOURCE AND

FEEDBACK RESISTANCE

Source and feedback resistances greater than 100 k

Ω will

magnify 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 since the circuit’s bandwidth and stability

can be adversely affected.

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

circuit bandwidth to 1/2

π R

pulse response from a 1 M

Ω 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.

10

90

100

10mV

5

µs

0%

Figure 36. Follower Pulse Response from 1 M

Ω

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

isolated from the board appears in Figure 37. Figure 38 shows

the response of the same circuit with a 1 pF feedback

capacitance. Typical differential input capacitance for the

AD795 is 2 pF.

10

90

100

10mV

5

µs

0%

Figure 37. Inverter Pulse Response with 1 M

Ω Source and

Feedback Resistance

10

90

100

10mV

5

µs

0%

Figure 38. Inverter Pulse Response with 1 M

Ω Source and

Feedback Resistance, 1 pF Feedback Capacitance

OVERLOAD ISSUES

Driving the amplifier output beyond its linear region causes

some sticking; recovery to normal operation is within 2

µs of the

input voltage returning within the linear range.

If either input is driven below the negative supply, the amplifier’s

output will be driven high, causing a phenomenon called phase

reversal. Normal operation is resumed within 30

µs of the input

voltage returning within the linear range.

Figure 39 shows the AD795’s input currents versus differential

input voltage. Picoamp level input current is maintained for

differential voltages up to several hundred millivolts. This

behavior is only important if the AD795 is in an open-loop

application where substantial differential voltages are produced.

–6

–5

–4

–3

–2

–1

0

1

2

3

4

5

6

DIFFERENTIAL INPUT VOLTAGE –

±Volts

Figure 39. Input Bias Current vs. Differential Input Voltage