10

to

Ω

20 Ω

V

(V+)

V

I

V

O

R

(S)

R

(L)

C

(L)

OPA320-Q1

14

OPA320-Q1, OPA2320-Q1

SLOS884B – SEPTEMBER 2014 – REVISED DECEMBER 2018

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Product Folder Links: OPA320-Q1 OPA2320-Q1

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Copyright © 2014–2018, Texas Instruments Incorporated

Feature Description (continued)

7.3.3 EMI Susceptibility And Input Filtering

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

operational amplifier, 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 OPAx320-Q1 operational amplifier family incorporates an internal input low-pass filter that reduces the

amplifiers response to EMI. Both common-mode and differential mode filtering are provided by the input filter.

The filter is designed for a cut-off frequency of approximately 580 MHz (–3 dB), with a roll-off of 20 dB per

decade.

7.3.4 Output Impedance

The open-loop output impedance of the OPAx320-Q1 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

130 dB (typical) 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 OPAx320-Q1 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 in

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

large capacitive loads. As mentioned previously, the OPAx320-Q1 have excellent capacitive load drive capability

for op amps with the bandwidth.

7.3.5 Capacitive Load and Stability

The OPAx320-Q1 are designed to be used in applications where driving a capacitive load is required. As with all

op amps, there may be specific instances where the OPAx320-Q1 can become unstable. The particular op amp

circuit configuration, layout, gain, and output loading are some of the factors to consider when establishing

whether an amplifier is stable in operation. An op amp in the unity-gain (1 V/V) buffer configuration and driving a

capacitive load exhibits a greater tendency to become unstable than an amplifier operated at a higher noise gain.

The capacitive load, in conjunction with the op amp output resistance, creates a pole within the feedback loop

that degrades the phase margin. The degradation of the phase margin increases as the capacitive loading

increases. When operating in the unity-gain configuration, the OPAx320-Q1 remain stable with a pure capacitive

load up to approximately 1 nF.

The equivalent series resistance (ESR) of some very large capacitors (C(L) > 1 µF) is sufficient to alter the phase

characteristics in the feedback loop such that the amplifier remains stable. Increasing the amplifier closed-loop

gain allows the amplifier to drive increasingly larger capacitance. This increased capability is evident when

observing the overshoot response of the amplifier at higher voltage gains; see Figure 32. One technique for

increasing the capacitive load drive capability of the amplifier operating in unity gain is to insert a small resistor

(R(S)), typically 10 Ω to 20 Ω, in series with the output, as shown in Figure 31.

This resistor significantly reduces the overshoot and ringing associated with large capacitive loads. A possible

problem with this technique is that a voltage divider is created with the added series resistor and any resistor

connected in parallel with the capacitive load. The voltage divider introduces a gain error at the output that

reduces the output swing. The error contributed by the voltage divider may be insignificant. For instance, with a

load resistance, R(L) = 10 kΩ and R(S) = 20 Ω, the gain error is only about 0.2%. However, when R(L) is

decreased to 600 Ω, which the OPAx320-Q1 are able to drive, the error increases to 7.5%.

Figure 31. Improving Capacitive Load Drive