How to stay out of trouble when designing with bridge sensors

June 20, 2014 // By Gustavo Castro and Scott Hunt, Analog Devices
Instrumentation amplifiers (in-amps) can condition the electrical signals generated by sensors, allowing them to be digitised, stored, or used to control processes.

The signal is typically small, so the amplifier may need to be operated at high gain. In addition, the signal may sit on top of a large common-mode voltage, or it may be embedded in a substantial dc offset. Precision in-amps can provide high gain, selectively amplifying the difference between the two input voltages while rejecting signals common to both inputs.

Wheatstone bridges are classic examples of this situation, but galvanic cells such as biosensors have similar characteristics. The bridge output signal is differential, so an in-amp is the preferred device for high-precision measurements. Ideally, the unloaded bridge output is zero, but this is true only when all four resistors are exactly equal. Consider a bridge built with discrete resistors, as shown in Figure 1. The worst case differential offset, VOS, is

[eq 1]

where VEX is the bridge excitation voltage and TOL is the resistor tolerance (in percent).

Figure 1. Wheatstone bridge offset.

For example, with 0.1% tolerance for each one of the individual elements and a 5-V excitation voltage, the differential offset can be as high as 5 mV. If a gain of 400 is required to achieve the desired bridge sensitivity, the offset becomes ±2V at the amplifier output. Assuming that the amplifier is powered by the same supply, and that its output can swing rail-to-rail, more than 80% of the output swing could be consumed by the bridge offset alone. As the industry trends to smaller supply voltages, this problem only gets worse.

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