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Occasionally an application requires an instrumentation amplifier to measure a floating source. Examples of floating sources are a battery powered voltage source, a source coupled through a transformer or capacitors, or a thermocouple. Instrumentation amplifiers are not able to measure floating sources, at least not for very long. This is because instrumentation amplifiers have bias currents that will charge up the floating source until it is outside the instrumentation amplifier’s common mode input range. In order to prevent this from happening, a high value resistor should be used to give the bias current a return path to the in amp’s power supplies. This resistor is tied to a voltage source within the In Amp’s input range, typically ground. Because In Amp bias currents are normally quite small, this bias current resistor can be quite large: typically 1MΩ or more. Note that bias current resistors are not required when both the sensor and the In Amp have a return path to ground. For example if a sensor is tied to earth ground and the In Amp’s power supply is also referenced to this same earth ground, no bias current resistor is needed.
