AD795 Analog Devices, AD795 Datasheet - Page 14

• Manufacturer Part Number: AD795
• Description: Low Power, Low Noise Precision FET Op Amp
• Manufacturer: Analog Devices

Specifications of AD795

• -3db Bandwidth: 1.6MHz
• Slew Rate: 1V/µs
• Vos: 100µV
• Ib: 1pA
• # Opamps Per Pkg: 1
• Input Noise (nv/rthz): 9nV/rtHz
• Vcc-vee: 8V to 36V
• Isy Per Amplifier: 1.5mA
• Packages: SOIC

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, R
mode capacitance, C
capacitance inherent to the AD795), form a pole that limits
circuit bandwidth to 1/2 π R
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.
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
isolated from the board appears in Figure 39. Figure 40 shows
Figure 38. Follower Pulse Response from 1 MΩ Source Resistance
100
0%
90
10
10mV
S
F
(including capacitance due to board and
and R
S
C
S
equal to 1 MΩ and the input pin
S
. Figure 38 shows the follower
S
, and input common-
5µs
Rev. C | Page 14 of 20
the response of the same circuit with a 1 pF feedback
capacitance. Typical differential input capacitance for the
AD795 is 2 pF.
Figure 39. Inverter Pulse Response with 1 MΩ Source and Feedback
Figure 40. Inverter Pulse Response with 1 MΩ Source and Feedback
100
100
0%
0%
10
10
90
90
10mV
10mV
Resistance, 1 pF Feedback Capacitance
Resistance
5µs
5µs