Difference Amplifier: Our goal is to "do more with less!"

The classic four-resistor difference amplifier is shown in Figure 1, but the performance of this circuit may not be as good as the designer would like. Starting from actual production design, this article discusses some of the disadvantages associated with discrete resistors, including gain accuracy, gain drift, AC common-mode rejection (CMR), and offset drift.

Figure 1. Classic discrete difference amplifier.

The transfer function of this amplifier circuit is:

If R1 = R3 and R2 = R4, Equation 1 simplifies to:

This simplification is useful for quickly estimating the expected signal, but these resistors are never exactly equal. In addition, resistors typically have low precision and high temperature coefficients, which can introduce significant errors into the circuit.

For example, using a good op amp and standard 1%, 100ppm/°C gain setting resistors, the initial gain error can be up to 2% and the temperature drift can be up to 200ppm/°C. To address this problem, one solution is to use a monolithic resistor network for precision gain setting, but such structures are bulky and expensive. In addition to low precision and significant temperature drift, most discrete differential op amp circuits have poor CMR and an input voltage range that is less than the supply voltage. In addition, monolithic instrumentation amplifiers will have gain drift because the internal resistor network of the preamplifier does not match the external gain setting resistors connected to the RG pin.

The best way to solve all of these problems is to use a difference amplifier with internal gain setting resistors, such as the AD8271. Typically, these products consist of a high-precision, low-distortion op amp and several trimmed resistors. These resistors can be connected to create a wide variety of amplifier circuits, including differential, non-inverting, and inverting configurations. The resistors on the chip can be connected in parallel to provide a wider range of options. Using on-chip resistors offers the designer several advantages over discrete designs.

Figure 2. Gain error vs. temperature—AD8271 versus discrete solution

In terms of circuit size, integrated circuits are much smaller than printed circuit boards (PCBs), so the corresponding parasitic parameters are also smaller, which is beneficial to AC performance. For example, the positive and negative inputs of the AD8271 op amp intentionally do not provide output pins. These nodes are not connected to the traces on the PCB, and the capacitance is kept low, thereby improving loop stability and optimizing common-mode rejection over the entire frequency range. See Figure 3 for a performance comparison.

Figure 3. CMRR vs. frequency—AD8271 vs. discrete solution CMRR

An important function of the difference amplifier is to reject the common-mode signal between the two inputs. Referring to Figure 1, if the resistors R1 to R4 are not perfectly matched (or the ratios of R1, R2 and R3, R4 are not matched when the gain is greater than 1), then part of the common-mode voltage will be amplified by the difference amplifier and appear at V _OUT as the effective differential voltage between V1 and V2 , which cannot be distinguished from the actual signal. If the resistors are not ideal, then part of the common-mode voltage will be amplified by the difference amplifier and appear at V _OUT as the effective differential voltage between V1 and V2 , which cannot be distinguished from the actual signal.

The ability of the difference amplifier to reject this portion of the voltage is called common-mode rejection. This parameter can be expressed as the common-mode rejection ratio (CMRR) or converted to decibels (dB). The resistor matching of the discrete solution is not as good as the laser-trimmed resistor matching in the integrated solution, which can be seen from the relationship between output voltage and CMV in Figure 4.

Figure 4. Output voltage vs. common-mode voltage—AD8271 vs. discrete solution

Assuming an ideal op amp, the CMRR is:

Where d is the gain of the difference amplifier and t is the resistor tolerance. Therefore, for unity gain and 1% resistors, the CMRR is 50V/V or about 34dB; with 0.1% resistors, the CMRR increases to 54dB. Even with an ideal op amp with infinite common-mode rejection, the overall CMRR is limited by the resistor matching. Some low-cost op amps have a minimum CMRR of 60 dB to 70 dB, making the error even worse.

Amplifiers generally perform well over their specified operating temperature range, but the temperature coefficients of external discrete resistors must be considered. For amplifiers with integrated resistors, the resistors can be drift trimmed and matched. The layout usually places the resistors close together so they drift together, reducing their offset temperature coefficients. In the discrete case, the resistors are spread out on the PCB and are not as well matched as the integrated solution, resulting in a worse offset temperature coefficient, as shown in Figure 5.

Figure 5. System offset vs. temperature—AD8271 vs. discrete solution

Whether discrete or monolithic, the four-resistor difference amplifier is widely used. Since only one device is placed on the PCB instead of multiple discrete components, the board can be built more quickly and efficiently, saving a lot of area.

To obtain a stable and production-worthy design, noise gain, input voltage range, and CMR (to 80dB or better) should be carefully considered. These resistors are made from the same low-drift thin-film material, thus providing excellent ratio matching over temperature.