Effects of Differential Input Clamping on Operational Amplifiers

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Effects of Differential Input Clamping on Operational Amplifiers [Copy link]

An op amp should have approximately zero voltage between its inputs, so these diodes will never be forward biased in a standard op amp circuit... or will they be?

Previously, we discussed the effects of the internal differential input clamping diodes on op amps when used as comparators (Details: Op Amps as Comparators, Is It Possible?). I posed the question – do these clamps affect op amp circuits?

An op amp should have approximately zero voltage between its inputs, so these diodes will never be forward biased in a standard op amp circuit... or will they be?

As a reminder, we are discussing some differential clamp diodes that may appear in some op amps, see Figure 1.

The effects of op amp circuits are often seen in basic non-inverting amplifier configurations, including a simple G=1 buffer amplifier.

Let’s look at a positive input step. The output cannot immediately follow the surge input voltage. If the input step is greater than 0.7V, D1 conducts, affecting the non-inverting input. As the op amp is slewing to its new output voltage, the current at the op amp input suddenly increases to some higher spike value, see Figure 2. Eventually, when the output “catches up” to the input, everything is fine again.

Many applications inherently process slow or band-limited signals, much slower than the slew rate of the op amp, so this situation will certainly not occur.

In some other applications, input current transients do not adversely affect circuit operation even if the input voltage changes rapidly. However, in some special cases, input current pulses can cause many problems. One notable case is a multiplexed data acquisition system.

The figure below shows a simplified example of such a system with only two input channels.

In this example, the multiplexer switches between channel 1 and channel 2, requiring the output of U1 to quickly switch from -5V to +5V. D1 is forward biased and the resulting input current transient passes through the multiplexer switch, discharging the voltage across C2. An R/C input filter is typically used to maintain a stable voltage during channel switching, but the current pulse partially discharges C2. Now, C2 needs more time to recharge to the correct input voltage, reducing the multiplexing rate and, therefore, accuracy.

The solution is to choose an op amp for U1 that does not have a differential clamp. FET input amplifiers such as the OPA140 have low input bias current (to reduce the burden on the MUX series resistor) and no differential input clamps, making them ideal for multiplexing inputs. The OPA827 performs well in most applications—FET inputs, very low noise, high speed, and fast settling. However, it has some differential input clamps, so the OPA827 may not be the best choice for an op amp multiplexer.