OPA320 datasheet(14/37 Pages) NSC

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OPA320

V+

OUT

OPA320, OPA2320

OPA320S, OPA2320S

SBOS513E – AUGUST 2010 – REVISED JUNE 2013

www.ti.com

FEEDBACK CAPACITOR IMPROVES

result of signal rectification associated with the

RESPONSE

internal semiconductor junctions. While all operational

amplifier pin functions can be affected by EMI, the

For optimum settling time and stability with high-

input pins are likely to be the most susceptible. The

impedance feedback networks, it may be necessary

OPA320 operational amplifier family incorporates an

to add a feedback capacitor across the feedback

internal

input

low-pass

filter

that

reduces

the

resistor, RF, as shown in Figure 35. This capacitor

amplifiers response to EMI. Both common-mode and

compensates for the zero created by the feedback

differential mode filtering are provided by the input

network

impedance

and

the

OPA320

input

filter. The filter is designed for a cut-off frequency of

capacitance (and any parasitic layout capacitance).

approximately 580MHz (–3dB), with a roll-off of 20dB

The effect becomes more significant with higher

per decade.

impedance networks.

OUTPUT IMPEDANCE

The open-loop output impedance of the OPA320

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

example, with 130dB (typ) of open-loop gain, the

output impedance is reduced in unity-gain to less

than 0.03

Ω. 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 OPA320 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

NOTE: Where CIN is equal to the OPA320 input capacitance

these frequencies the output also becomes capacitive

(approximately 9pF) plus any parasitic layout capacitance.

as a result of parasitic capacitance. This in turn

prevents the output impedance from becoming too

Figure 35. Feedback Capacitor Improves

high, which can cause stability problems when driving

Dynamic Performance

large capacitive loads. As mentioned previously, the

OPA320 has excellent capacitive load drive capability

It is suggested that a variable capacitor be used for

for an op amp with its bandwidth.

the feedback capacitor because input capacitance

may vary between op amps and layout capacitance is

CAPACITIVE LOAD AND STABILITY

difficult to determine. For the circuit shown in

The OPA320 is designed to be used in applications

Figure 35, the value of the variable feedback

where driving a capacitive load is required. As with all

capacitor should be chosen so that the input

op amps, there may be specific instances where the

resistance times the input capacitance of the OPA320

OPA320 can become unstable. The particular op amp

(typically 9pF) plus the estimated parasitic layout

circuit configuration, layout, gain, and output loading

capacitance equals the feedback capacitor times the

are some of the factors to consider when establishing

feedback resistor:

whether an amplifier is stable in operation. An op

RIN × CIN = RF × CF

amp in the unity-gain (+1V/V) buffer configuration and

Where:

driving a capacitive load exhibits a greater tendency

to become unstable than an amplifier operated at a

CIN is equal to the OPA320 input capacitance

higher noise gain. The capacitive load, in conjunction

(sum of differential and common-mode) plus the

with the op amp output resistance, creates a pole

layout capacitance.

within the feedback loop that degrades the phase

The

capacitor

value

can

adjusted

until

margin.

The

degradation

the

phase

margin

optimum performance is obtained.

increases as the capacitive loading increases. When

operating in the unity-gain configuration, the OPA320

EMI SUSCEPTIBILITY AND INPUT FILTERING

remains stable with a pure capacitive load up to

Operational

amplifiers

vary

susceptibility

approximately 1nF.

electromagnetic interference (EMI). If conducted EMI

enters

the

operational

amplifier,

the

offset

observed at the amplifier output may shift from the

nominal value while EMI is present. This shift is a

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