Chapter 3 Signal Connections© National Instruments Corporation 3-9 AT-MIO-16 User Manual+-+FloatingSignalSourceInput MultiplexersInstrumentationAmplifierV m MeasuredVoltage-VS-+I/O ConnectorAT-MIO-16 Board in DIFF Configuration3571746818AIGND1, 2100 kΩBiasCurrentReturnPathsACH<8..15>ACH<0..7>19AISENSEFigure 3-4. Differential Input Connections for Floating Signal SourcesThe 100 kΩ resistors shown in Figure 3-4 create a return path to ground for the bias currents ofthe instrumentation amplifier. If there is no return path, the instrumentation amplifier biascurrents charge up stray capacitances, resulting in uncontrollable drift and possible saturation inthe amplifier. Typically, resistors from 10 kΩ to 100 kΩ are used.A resistor from each input to ground, as shown in Figure 3-4, produces bias current return pathsfor an AC-coupled input signal. This solution, although necessary for AC-coupled signals,lowers the input impedance of the analog input channel. In addition, the input offset current ofthe instrumentation amplifier contributes a DC offset voltage at the input. The amplifier has amaximum input offset current of ±15 nA and a typical offset current drift of ±20 pA/°C.Multiplied by the 100 kΩ resistor, this current contributes a maximum offset voltage of 1.5 mVand a typical offset voltage drift of 2 μV/°C at the input. Keep this in mind when you observeDC offsets with AC-coupled inputs.If the input signal is DC-coupled, you need only the resistor that connects the negative signalinput to ground. This connection does not lower the input impedance of the analog inputchannel.
