Amplifier circuit design, how critical is the DC loop?

“Have you ever had such an experience? Designing a circuit in a hurry and ignoring some fundamental issues results in the circuit not functioning as expected. . . One of the most common problems in AC-coupled op amp or instrumentation amplifier circuit applications is that there is no DC return for bias current. Today, I will discuss this problem for everyone, and propose a super practical solution.

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Have you ever had such an experience? Designing a circuit in a hurry and ignoring some fundamental issues results in the circuit not functioning as expected. . . One of the most common problems in AC-coupled op amp or instrumentation amplifier circuit applications is that there is no DC return for bias current. Today, I will discuss this problem for everyone, and propose a super practical solution.

Operational Amplifiers: How to Provide a DC Return for Bias Current

× Error example

Figure 1 Wrong AC-coupled op amp circuit

In Figure 1, a capacitor is connected in series to the non-inverting (+) input of an op amp. This AC coupling is an easy way to isolate the DC voltage from the input voltage (VIN) and is especially useful in high gain applications. At higher gains, even a small DC voltage at the amplifier input can affect the dynamic range of the op amp and may even cause the output to saturate. However, capacitive coupling into the high impedance input without providing a DC path for the current in the positive input presents some problems.
　　
Input bias current flows through the coupling capacitor, charging it until the common-mode voltage rating of the amplifier’s input circuit is exceeded or the output limit is exceeded. Depending on the polarity of the input bias current, the capacitor charges either in the direction of the positive supply voltage or in the direction of the negative supply voltage. This bias voltage is amplified by the closed-loop DC gain of the amplifier.

This process may be longer.For example, for an amplifier with a field effect Transistor (FET) input with a bias current of 1 pA coupled through a 0.1-µF capacitor, the IC charge rate I/C is

10-12/10-7 = 10 μV/sec

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

√ Display correctly

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. In order to minimize the offset voltage caused by the input bias current, when using a bipolar op amp, consider the matching problem of the two input terminals of the op amp, and usually set R1 to the parallel value of R2 and R3.

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

Note, however, that this resistor will always introduce some noise to the circuit, so there is a trade-off between the input impedance of the circuit, the size of the input coupling capacitor required, and the Johnson noise introduced by the resistor. Typical resistance values ​​are generally between 100,000 Ω and 1 MΩ.

Instrumentation Amplifiers: How to Provide a DC Return for Bias Current

× Error example

Figure 3 shows the in-amp circuit AC-coupled through two capacitors, also without a return loop for the input bias current. This problem is commonly seen in instrumentation amplifier circuits that operate from a dual supply (Figure 3a) and a single supply (Figure 3b).

Figure 3 Wrong AC Coupled Instrumentation Amplifier Circuit

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

Figure 4 Wrong Transformer-Coupled Instrumentation Amplifier Circuit

√ Display correctly

Figures 5 and 6 show simple solutions for such circuits. A high value resistor (RA, RB) is added between each input and ground. This is a simple and practical solution for dual-supply instrumentation amplifier circuits. The resistor provides a discharge path for the input bias current. In the dual-supply example, both inputs are referenced to ground. In the single-supply example, the input can be either referenced to ground (VCM ground) or a bias voltage, typically half the maximum input voltage range.

Figure 5 Proper Method for Input Coupling of Instrumentation Amplifier Transformers

The same principle can be used for the transformer-coupled input (Figure 5), unless the transformer secondary winding has a center tap, which can either be grounded or connected to VCM. In these circuits, there is a small offset voltage error due to resistor and/or input bias current mismatch. To minimize such errors, connect another resistor (but still larger than the differential source resistance) between the two inputs of the instrumentation amplifier with a resistance value approximately one-tenth of the two resistors (but still large compared to the differential source resistance). thus bridging the two resistors).