Low-power, low-cost, differential amplifier with high CMRR

Analog Devices ADA4805-2

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Many applications require special differential amplifiers that achieve both high performance and low power dissipation. A simple solution is to use a dual non-inverting precision amplifier with a resistive gain network, but the DC common-mode level of the differential outputs will depend upon the common-mode voltage at the input. Figure 1 shows the ADA4805­2 high-performance dual amplifier configured as an inverting differential amplifier. This low­noise circuit operates with common-mode gain of +1 from the two non-inverting inputs to the outputs, enabling easy common-mode control for single-supply operation. The low quiescent current (500 µA per amplifier) suits the circuit for low-power, high-resolution data-conversion systems.

Inverting differential amplifier configuration.
Figure 1. Inverting differential amplifier configuration.

This circuit offers a combination of low distortion and low quiescent current. The dual op amp solution results in lower system cost, while the differential amplifier topology yields better performance.

High-resolution ADCs often have differential inputs to reduce common-mode noise and harmonic distortion. Therefore, single-ended input voltages must be converted to differential input voltages before they can be useful for ADCs. This circuit can be used to convert a single­ended signal into a differential signal to drive the ADCs while the common-mode voltage can be set by the ADC’s reference. 

The transformer converts the single-ended signal into a differential signal. With a system gain of –1, the differential output can drive the ADC. The common-mode output voltage is determined by the voltage VREF at two non-inverting inputs.

To eliminate the DC common-mode at the inputs, add a series capacitor to each input, or a transformer as shown in Figure 1. Also, the differential input signals can be applied directly to IN+ and IN– pins. In this case, the gain resistors become part of the input resistance for the source.

The differential-mode frequency response plot was taken by applying a differential input voltage at the two input terminals (IN+ and IN–).

Frequency response for differential and common mode signals.
Figure 2. Frequency response for differential and common mode signals.

The gain resistors become part of the input resistance for the source. This provides better noise performance than the non-inverting con­figuration but limits the flexibility in setting the input impedance separately from the gain.

For the common-mode frequency response plot, the common-mode voltage is applied at two non-inverting input terminals (VREF) with IN+ and IN– tied together.

Materials on the topic

  1. Datasheet Analog Devices ADA4805-2

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