Data Acquisition

Increased noise immunity is also achieved using a balanced system, since induced noisevoltages appearing on the signal wires, are equal and should be cancelled out by the differentialamplifier measurement.Bias resistors, connected between each input lead and the ground reference of themeasurement system, as shown in Figure 3.28, provide a DC return path for bias currentsfrom the inputs of the instrumentation amplifier to the reference ground.Figure 3.28Differential measurement of an ungrounded signal sourceWhere the signal contains both AC and DC components (i.e. DC coupled) and thesignal source has low impedance, only one bias resistor is required to be connected betweenthe negative input and the ground reference. If the source impedance is relativelyhigh compared to the input impedance of the instrumentation amplifier, then the imbalancecaused by using a single bias resistor could lead to erroneous results. Therefore,for high source impedances both input bias resistors should be used.For input signals, which contain no DC component (i.e. AC coupled), both bias resistorsare required.The bias resistors should be large enough to allow the source to float with respect to themeasurement system ground and not to load the signal source (i.e. much greater than thesource impedance), but small enough to keep the voltage at each input terminal within theinput stage common mode voltage range of the measurement system. Bias resistorsbetween 10 kΩ and 100 kΩ are typically used for low impedance sources such asthermocouples or when connecting the outputs of signal conditioning modules to dataacquisition boards.3.4.10 System isolationTo allow the measurement of signals that contain large common mode voltages, specialhardware and measurement techniques are used. This typically involves isolating themeasurement system from the ground reference so that signal lines, such as amplifiers,commonly used as a measurement reference, become a floating reference point.System isolation can be carried out in the following ways:• Using isolation transformers to reject the common mode voltage appearing onthe signal lines.