OPA322

V+

V

OUT

V

IN

R

IN

R

C

=R

´

C

´

IN

IN

F

F

R

F

C

L

C

IN

C

IN

C

F

OPA322, OPA322S

OPA2322, OPA2322S

OPA4322, OPA4322S

www.ti.com

SBOS538C

– JANUARY 2011 – REVISED NOVEMBER 2011

FEEDBACK CAPACITOR IMPROVES RESPONSE

For optimum settling time and stability with high-impedance feedback networks, it may be necessary to add a

zero created by the feedback network impedance and the OPA322 input capacitance (and any parasitic layout

capacitance). The effect becomes more significant with higher impedance networks.

Figure 28. Feedback Capacitor Improves Dynamic Performance

It is suggested that a variable capacitor be used for the feedback capacitor because input capacitance may vary

between op amps and layout capacitance is difficult to determine. For the circuit shown in Figure 28, the value of

the variable feedback capacitor should be chosen so that the input resistance times the input capacitance of the

OPA322 (typically 9 pF) plus the estimated parasitic layout capacitance equals the feedback capacitor times the

feedback resistor:

Where:

capacitance.

The capacitor value can be adjusted until optimum performance is obtained.

EMI SUSCEPTIBILITY AND INPUT FILTERING

Operational amplifiers vary in susceptibility to electromagnetic interference (EMI). If conducted EMI enters the

device, the dc offset observed at the amplifier output may shift from the nominal value while EMI is present. This

shift is a result of signal rectification associated with the internal semiconductor junctions. While all operational

amplifier pin functions can be affected by EMI, the input pins are likely to be the most susceptible. The OPA322

operational amplifier family incorporates an internal input low-pass filter that reduces the amplifier response to

EMI. Both common-mode and differential mode filtering are provided by the input filter. The filter is designed for a

cutoff frequency of approximately 580 MHz (

–3 dB), with a roll-off of 20 dB per decade.

OUTPUT IMPEDANCE

The open-loop output impedance of the OPA322 common-source output stage is approximately 90

Ω. When the

op amp is connected with feedback, this value is reduced significantly by the loop gain. For each decade rise in

the closed-loop gain, the loop gain is reduced by the same amount, which results in a ten-fold increase in

effective output impedance. While the OPA322 output impedance remains very flat over a wide frequency range,

at higher frequencies the output impedance rises as the open-loop gain of the op amp drops. However, at these

frequencies the output also becomes capacitive as a result of parasitic capacitance. This characteristic, in turn,

prevents the output impedance from becoming too high, which can cause stability problems when driving large

capacitive loads. As mentioned previously, the OPA322 has excellent capacitive load drive capability for an op

amp with its bandwidth.

Copyright

© 2011, Texas Instruments Incorporated

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Product Folder Link(s): OPA322 OPA322S OPA2322 OPA2322S OPA4322 OPA4322S