OPA350 Datasheet, PDF (20/40 Pages) Burr-Brown (TI) – High-Speed, Single-Supply, Rail-to-Rail OPERATIONAL AMPLIFIERS MicroAmplifier ™ Series

OPA350, OPA2350, OPA4350
SBOS099D – SEPTEMBER 2000 – REVISED DECEMBER 2015
www.ti.com
Typical Applications (continued)
8.2.3 Adding a Feedback Capacitor to Improve Response
For optimum settling time and stability with high-impedance feedback networks, it may be necessary to add a
feedback capacitor across the feedback resistor, R_F, as shown in Figure 32. This capacitor compensates for the
zero created by the feedback network impedance and the input capacitance of the OPA350 (and any parasitic
layout capacitance). The effect becomes more significant with higher impedance networks.
C_F
R_IN
R_F
V_IN
V+
R_IN • C_IN = R_F • C_F
C_IN
O PA 350
C_IN
V_OUT
C_L
Where C_IN is equal to the OPA350’s input
capacitance (approximately 9pF) plus any
parasitic layout capacitance.
Figure 32. Feedback Capacitor Improves Dynamic Performance
A variable capacitor can be used for the feedback capacitor, because input capacitance may vary between
operational amplifiers and layout capacitance is difficult to determine. For the circuit shown in Figure 32, the
value of the variable feedback capacitor should be chosen so that the input resistance times the input
capacitance of the OPA350 (typically 9 pF) plus the estimated parasitic layout capacitance equals the feedback
capacitor times the feedback resistor:
R_IN × C_IN = R_F × C_F
where
• C_IN is equal to the input capacitance of the OPA350 (sum of differential and common-mode) plus the layout
capacitance.
(3)
The capacitor can be varied until optimum performance is obtained.
8.2.4 Two Op-Amp Instrumentation Amplifier With Improved High-Frequency Common-Mode Rejection
The OPAx350 is well suited for high input impedance applications such as an instrumentation amplifier. The two
amplifier configuration shown in Figure 33 rejects any common mode signals and senses the small differential
input voltage developed by the resistive bridge. The voltage reference sets the output to 2.5 V when the
differential signal developed by the bridge is zero. The high common mode rejection versus frequency response
of the OPAx350, rejects and common mode noise that may be coupled into the bridge circuit from the bridge
excitation source. The gain of the circuit is determined by R_G according to the equation shown in Figure 33.
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