AD795 View Datasheet(PDF) - Analog Devices
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Description
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AD795 Datasheet PDF : 20 Pages
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AD795
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.
the response of the same circuit with a 1 pF feedback
capacitance. Typical differential input capacitance for the
AD795 is 2 pF.
10mV
5µs
100
90
10
0%
10mV
5µs
100
Figure 39. Inverter Pulse Response with 1 MΩ Source and Feedback
90
Resistance
10mV
5µs
100
90
10
0%
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
10
0%
Figure 40. Inverter Pulse Response with 1 MΩ Source and Feedback
Resistance, 1 pF Feedback Capacitance
Rev. C | Page 14 of 20
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