Column: Design problem solvers


Figure 1: Typical differential amplifier circuit


Conventional resistors don’t have constant values in


practice because of the mechanical loads and effects of temperature they are subjected to. Depending on the requirement, resistors with different tolerances or matched resistor pairs or networks may be used, which for the most part are made using thin-film technology and offer precise ratiometric stability. With these networks (for example, the LT5400 quad matched resistor network), the overall CMRR of the amplifier circuit can be dramatically improved. The LT5400 resistor network guarantees a CMRR up to two times better when compared with discrete resistor solutions, due to its outstanding matching over the entire temperature range, especially when used in combination with differential amplifier circuits. Thus, the LT5400 offers 0.005% matching, which results in a CMRRR


of 86dB.


However, the total common-mode rejection ratio of amplifier circuits (CMRRTotal


) is formed by the combination


of the resistor CMRR and the common-mode rejection ratio of the op-amp, CMRROP


. For differential amplifiers, this is


Accuracy requirements The external resistor circuit must be selected carefully, based on the requirements for accuracy of the differential amplifier circuit, to achieve that high system performance.


calculated using Equation 3: (3)


For a typical CMRROP


of 112dB, as offered by the LT1468,


and a gain of G = 1 with the LT5400, a value of 85.6dB is determined for CMRRTotal


. Alternatively, integrated differential amplifiers such as the


LTC6363 can be used, a solution that already contains the actual amplifier and optimally-matched resistors on a chip. It eliminates nearly all the problems mentioned here, and offers maximum accuracy at CMRR values over 90dB.


Figure 2: Differential


amplifier circuit with LT5400


www.electronicsworld.com September/October 2020 11


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