In applications such as electromagnetic-flow (EM flow) meters and biopotential measurements, small differential signals are in series with much larger differential offsets. These offsets typically limit the gain that the circuit can take in the front-end design, which in turn impacts the overall dynamic range. The gain limitation is even more challenging when lower supply voltages are used—for example, in battery-powered signal chains.
One solution to address this large differential offset issue is to use an ac-coupled measurement signal chain. A typical ac-coupled signal chain would include a low-gain instrumentation amplifier, followed by a high-pass filter and additional gain stages. In most applications, it’s preferable to get as much gain as possible at the first stage, as this helps improve the referred-to-input (RTI) noise of the following gain stages in the signal chain.