How to provide a DC return path for bias current? Correct vs. Incorrect Example

In Figure 1, a capacitor is connected in series with the noninverting (+) input of an op amp. This AC coupling is a simple way to isolate DC voltages from the input voltage ( V _{I N} ) and is particularly useful in high-gain applications. At high gains, even a small DC voltage at the amplifier input can affect the dynamic range of the op amp and can even cause the output to saturate. However, capacitive coupling into a high-impedance input without providing a DC path for the current in the positive input can cause problems.

Figure 1 Error AC coupled operational amplifier circuit

The input bias current flows through the coupling capacitor, charging it until it exceeds the rated common-mode voltage of the amplifier input circuit or exceeds the output limit. Depending on the polarity of the input bias current, the capacitor charges either toward the positive supply voltage or toward the negative supply voltage. This bias voltage is amplified by the closed-loop DC gain of the amplifier.

This process can be lengthy. For example, for an amplifier with field-effect transistor (FET) inputs, if its bias current is 1 pA and coupled through a 0.1-μF capacitor, its IC charge rate, I/C, is

10 ^{- 12} / 10 ^{- 7} = 10 μV/sec

The total output drift is 600 μV/min. If the gain is 100, the output drift is 0.06 V/min. It can be seen that if an AC-coupled oscilloscope is used for a short test, this problem may not be detected, and the circuit will fail after several hours. In short, it is very important to avoid this problem.

Figure 2 shows a simple solution. In this example, a resistor is connected between the input of the op amp and ground, providing a return path for the input bias current. To minimize the offset voltage caused by the input bias current, when using a bipolar op amp, consider the matching of the two inputs of the op amp, and usually set R1 to the parallel value of R2 and R3.

Figure 2: Correct method for AC coupling at the input of a dual-supply op amp

However, it should be noted that this resistor will always contribute some noise to the circuit, so a trade-off needs to be made between the circuit input impedance, the size of the input coupling capacitor required, and the Johnson noise introduced by the resistor. Typical resistor values ​​are generally between 100,000 Ω and 1 MΩ.

Figure 3 shows an instrumentation amplifier circuit that is AC-coupled through two capacitors and does not provide a return path for the input bias current. This problem is common in instrumentation amplifier circuits that use dual supplies (Figure 3a) and single supplies (Figure 3b).

Figure 3 Error AC-coupled instrumentation amplifier circuit

As shown in Figure 4, this problem can also occur in circuits utilizing transformer coupling if a DC return path to ground is not provided in the transformer secondary circuit.

Figure 4 Error Transformer coupled instrumentation amplifier circuit

Figure 5. Correct way to couple the instrumentation amplifier transformer input

The same principle can be applied to transformer-coupled inputs (Figure 5), except that the transformer secondary winding has a center tap that can be connected to either ground or V _CM . In these circuits, there is a small offset voltage error due to resistor and/or input bias current mismatches. To minimize this error, connect another resistor with a value about one-tenth that of the two resistors (but still large compared to the differential source resistance) between the two inputs of the in-amp (thereby bridging the two resistors).

Figure 6 High-value resistors between each input and ground provide the required bias current return path

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