To improve the common-mode rejection ratio of the differential amplifier, the choice of resistance is very important

“In various application fields, a differential amplifier circuit is required when using analog technology, as shown in Figure 1. For example, measurement technology, depending on its application, may require extremely high measurements. In order to achieve this, it is important to minimize typical error sources (such as offset and gain errors, as well as noise, tolerance, and drift) as much as possible. For this, a high operational amplifier is required. The choice of external components of the amplifier circuit is equally important, especially the resistors. They should have matching ratios and cannot be chosen arbitrarily.

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In various application fields, a differential amplifier circuit is required when using analog technology, as shown in Figure 1. For example, measurement technology, depending on its application, may require extremely high measurements. To achieve this, it is important to minimize typical error sources (such as offset and gain errors, as well as noise, tolerance, and drift). For this, a high operational amplifier is required. The choice of external components of the amplifier circuit is equally important, especially the resistors. They should have matching ratios and cannot be chosen arbitrarily.

Figure 1. Traditional differential amplifier circuit.

Ideally, the resistors in the differential amplifier circuit should be selected carefully, and their ratios should be the same (R2/R1 = R4/R3). Any deviation in these ratios will result in undesirable common mode errors. The ability of a differential amplifier to suppress this common-mode error is expressed in the common-mode rejection ratio (CMRR). It shows how the output voltage changes with the same input voltage (common mode voltage). In this case, the output voltage should not be changed because it only depends on the difference (CMRR) between the two input voltages; however, the actual situation will be different in actual use. CMRR is an important characteristic of a differential amplifier circuit, usually expressed in dB.

For the differential amplifier circuit shown in Figure 1, CMRR depends on the amplifier itself and the externally connected resistance. For the latter, the resistance-dependent CMRR is denoted by the subscript “R” in the following parts of this article, and is calculated using the following formula:

For example, in an amplifier circuit, the required gain G = 1 and using a resistor with a tolerance of 1% and a matching of 2% produces a common-mode rejection ratio of

or

At 34 dB, CMRRR is relatively low. In this case, even if the amplifier has a very good CMRR, it cannot achieve high, because the link always depends on its poor link. Therefore, for precision measurement circuits, resistors must be selected very carefully.

In actual use, the resistance of traditional resistors is not constant. They are affected by mechanical load and temperature. According to different requirements, resistors or matched resistor pairs (or networks) with different tolerances can be used, most of which are manufactured using thin film technology and have high ratio stability. Using these matched resistor networks (such as the LT5400 four-channel matched resistor network) can greatly increase the overall CMRR of the amplifier circuit. The LT5400 resistor network has excellent matching over the entire temperature range, and it is better matched when used in combination with a differential amplifier circuit, thus ensuring that the CMRR is twice as high as that of discrete resistors.

Figure 2. Differential amplifier circuit with LT5400.

The LT5400 provides 0.005% matching, resulting in a CMRRR of 86 dB. However, the total mode rejection ratio (CMRRTotal) of the amplifier circuit is composed of the combination of the resistance CMRR and the common mode rejection ratio CMRROP of the operational amplifier. For a differential amplifier, it can be calculated using Equation 3:

For example, the typical value of CMRROP provided by LT1468 is 112 dB, the gain of LT5400 is G = 1, and the value of CMRRTotal is 85.6 dB.

Or, you can use an integrated differential amplifier, such as the LTC6363. This type of amplifier has built-in amplifiers and matching resistors in a single chip. It almost eliminates all the above-mentioned problems and is also available with a CMRR value of more than 90 dB.

in conclusion

The external resistor circuit must be carefully selected according to the requirements of the differential amplifier circuit in order to achieve the high performance of the system. Or, you can use an integrated differential amplifier, such as the LTC6363 that integrates matching resistors in a single chip.