AD795_02 Datasheet, PDF (11/16 Pages) Analog Devices – Low Power, Low Noise Precision FET Op Amp

AD795
AC RESPONSE WITH HIGH VALUE SOURCE AND
FEEDBACK RESISTANCE
Source and feedback resistances greater than 100 kW 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.
In a follower, the source resistance, R_S, and input common-
mode capacitance, C_S (including capacitance due to board and
capacitance inherent to the AD795), form a pole that limits
circuit bandwidth to 1/2 p R_SC_S. Figure 35 shows the follower
pulse response from a 1 MW 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.
10mV
5m s
100
90
10
0%
Figure 35. Follower Pulse Response from 1 MW
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 R_F and R_S equal to 1 MW, and the input pin
isolated from the board appears in Figure 36. Figure 37 shows
the response of the same circuit with a 1 pF feedback
capacitance. Typical differential input capacitance for the
AD795 is 2 pF.
10mV
5m s
100
90
10mV
5m s
100
90
10
0%
Figure 37. Inverter Pulse Response with 1 MW 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 ms 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 ms of the input
voltage returning within the linear range.
Figure 38 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.
10^–4
10^–5
–I_N
+I_N
10^–6
10^–7
10^–8
10^–9
10^–10
10^–11
10^–12
10^–13
10^–14
–6 –5 –4 –3 –2 –1 0 1 2 3 4 5 6
DIFFERENTIAL INPUT VOLTAGE – ±Volts
Figure 38. Input Bias Current vs. Differential Input Voltage
10
0%
Figure 36. Inverter Pulse Response with 1 MW Source and
Feedback Resistance
REV. B
–11–