To improve the common mode rejection ratio of the differential amplifier, the choice of resistors is the key!

“In a variety of applications, using analog techniques requires the use of differential amplifier circuits, as shown in Figure 1. Measurement techniques, for example, may require extremely high measurement accuracy, depending on their application. To achieve this accuracy, it is critical to minimize typical error sources such as offset and gain errors, as well as noise, tolerance, and drift. For this, a high-precision operational amplifier is required. The selection of external components for the amplifier circuit is equally important, especially the resistors, which should have matching ratios and not be arbitrarily chosen.

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In a variety of applications, using analog techniques requires the use of differential amplifier circuits, as shown in Figure 1. Measurement techniques, for example, may require extremely high measurement accuracy, depending on their application. To achieve this accuracy, it is critical to minimize typical error sources such as offset and gain errors, as well as noise, tolerance, and drift. For this, a high-precision operational amplifier is required. The selection of external components for the amplifier circuit is equally important, especially the resistors, which should have matching ratios and not be arbitrarily chosen.

Figure 1. Traditional differential amplifier circuit.

Ideally, the resistors in the differential amplifier circuit should be carefully chosen to have the same ratio (R2/R1 = R4/R3). Any deviation in these ratios will result in undesirable common mode errors. The ability of a differential amplifier to reject this common-mode error is expressed as the common-mode rejection ratio (CMRR). It represents how the output voltage varies with the same input voltage (common mode voltage). In the best case, the output voltage should not change as it only depends on the difference between the two input voltages (maximum CMRR); however, in actual use things will be different. CMRR is an important characteristic of differential amplifier circuits and is usually expressed in dB.

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

For example, in an amplifier circuit, the desired gain G = 1 and the use of 1% tolerance, 2% matched resistors yields a common mode rejection ratio of

or

At 34 dB, CMRR_R relatively low. In this case, even if the amplifier has very good CMRR, high accuracy cannot be achieved because the accuracy of the link will always depend on its least accurate link. Therefore, for precision measurement circuits, the resistors must be chosen very precisely.

In actual use, the resistance of traditional resistors is not constant. They are affected by mechanical load and temperature. Depending on the requirements, resistors or matched resistor pairs (or networks) with different tolerances can be used, most of which are fabricated using thin-film technology and have precise ratio stability. Using these matched resistor networks, such as the LT5400 quad matched resistor network, can greatly improve the overall CMRR of the amplifier circuit. The LT5400 resistor network is well matched over temperature, and even better when used in conjunction with a differential amplifier circuit, ensuring a two-fold improvement in CMRR over discrete resistors.

Figure 2. Differential amplifier circuit with LT5400.

The LT5400 provides 0.005% matching accuracy, resulting in CMRR_R up to 86 dB. However, the overall mode rejection ratio (CMRR of the amplifier circuit)_Total) by the resistor CMR_R and op amp common mode rejection ratio CMRR_OP composition of. For a differential amplifier, it can be calculated using Equation 3:

For example, the CMRR provided by the LT1468_OP 112 dB typical, with a gain of G = 1 using the LT5400, its CMRR_Total is 85.6 dB.

Alternatively, an integrated differential amplifier such as the LTC6363 can be used. This amplifier has built-in amplifier and best matched resistors in a single chip. It virtually eliminates all of the above problems, yet also provides maximum accuracy with CMRR values ​​above 90 dB.

in conclusion

The external resistor circuit must be carefully selected according to the accuracy requirements of the differential amplifier circuit in order to achieve high performance of the system. Alternatively, an integrated differential amplifier such as the LTC6363 with matched resistors in a single chip can be used.

LT5400

• Excellent matching performance
• Class A: 0.01% match accuracy
• Class B: 0.025% match accuracy
• 0.2ppm/ºC matched temperature drift
• ±75V operating voltage (±80V absolute maximum)
• 8ppm/ºC absolute resistance temperature drift
• Long term stability: • C55ºC to 150ºC operating temperature range
• 8-pin MSOP package