Let me tell you a low-cost instrumentation amplifier design method~

Many low-cost instrumentation amplifiers offer bandwidth, DC precision, and low power that can meet all system requirements. Another benefit of using an instrumentation amplifier is that the user does not need to build their own differential amplifier, thereby eliminating many high-cost discrete components. This article will present a simple method to build a low-cost instrumentation amplifier and optimize its performance. In addition, the cost and performance of this solution are comparable to those of a monolithic instrumentation amplifier.

Figure 1 details the proposed precision system design that allows the user to measure differential signals in the presence of high common-mode voltages. The circuit includes an input buffer, an ADC driver, and a voltage reference. The buffer drives the reference pin of the instrumentation amplifier and converts the single-ended output to a differential output. The circuit has a very high input common-mode voltage range. It can handle common-mode voltages up to ±270 V (with ±15 V supplies), almost 20 times the supply voltage in both the positive and negative directions, which is critical for motor control applications. In addition, the inputs are protected against common-mode or differential-mode transients up to ±500 V.

This application uses ±5 V supplies, which allows the input voltage to have a ±80 V common-mode range.

The differential output is determined by the following formula:

The common-mode output is set by the following equation:

The benefit of this circuit is that the dc differential accuracy is determined by the AD629 difference amplifier and the AD8421 instrumentation amplifier, not by the op amp or the external 10 kΩ resistor. In addition, this circuit takes advantage of the instrumentation amplifier's precise control of its output voltage relative to its reference voltage. Although the op amp's dc performance and resistor matching will affect the dc common-mode output accuracy, these errors are likely to be rejected by the next device in the signal chain, so their impact on the overall system accuracy will be minimal.

For best ac performance, an op amp with high bandwidth and high slew rate is recommended. The op amp chosen in this circuit is the ADA4807.

To avoid parasitic capacitance that can destabilize the ADA4807, the trace length from the resistor to the inverting input should be as short as possible. If a longer trace must be used, use a lower value resistor.

High-performance ADCs typically operate from a single 5 V supply and have their own reference voltage. This reference voltage is used as the common-mode voltage for the differential output, eliminating the need for a reference voltage source. Therefore, its output is ratiometric with the ADC, which means that any changes in the ADC's V _REF will not affect the performance of the system.

The ability of this difference amplifier to reject common-mode voltages depends on the ratio matching of the trimmed resistors inside the AD629 difference amplifier. Therefore, it is better than an instrumentation amplifier using discrete amplifiers.

For discrete amplifiers using 0.1% external resistors, the CMR is limited to 54 dB. Instrumentation amplifiers integrate precision laser-trimmed resistors, enabling systems with CMRs of 80 dB or better. These resistors are made from the same low-drift thin-film material, providing excellent ratio matching over temperature.

The ADC can be powered from a single 5V supply with a 2.5V low impedance voltage source on the reference pin. This sets the output to midsupply and raises the common-mode voltage seen at the ADC input.

The oscilloscope output waveforms are shown in Figure 2. The gain of both instrumentation amplifiers is 1. V _IN is a 1 V pp 10 kHz sine wave on a large common-mode voltage. V _OUT + and V _OUT – are ±0.5 V pp sine and cosine waves, respectively. V _{OUT _diff is the 1 V pp differential output voltage, which is V IN} after the common-mode signal is removed .

The gain of the instrumentation amplifier can be increased by adding a resistor, RG:

This circuit can also be used in power-sensitive applications. With a total quiescent current of 5 mA and a 5-V dual supply, it consumes only about 50 mW, which is a 50% power saving compared to other solutions using basic ADC drivers (for example, the AD8138 and AD8131 differential driver amplifiers) or discrete amplifiers.