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More Common-Mode Tips
Jon Titus, Contributing Writer -- Design News, October 1, 2009
The previous tips column, "Reject Common-Mode Signals," described how instrumentation amplifiers can reduce the effects of common-mode signals or noise. This column offers two tips you also should consider when you plan to use an in-amp.
1. Match common-mode range and output. Although an in-amp will reject common-mode signals, it has limits. To help designers, in-amp manufacturers plot an "envelope" that defines a safe operating area for given gain, common-mode-signal, power-supply and output voltages. Four plots in the data sheet for a Texas Instruments' INA118 device, for example, show results for dual-supply voltages of ±15V, ±5V, +5V and +3V. Operating outside the envelope won't necessarily damage an in-amp, but it can lead to an incorrect output for a given input.
The diagram below shows the envelope for an INA118 device with a ±5-V supply, a gain set for 10x, and a reference of 0V. In this case, if you have a 2.0-V common-mode voltage, the in-amp will reliably produce an output between about -5 and +4.2V. The 10x gain means your sensor can create a signal between -500 and +420 mV, or more conservatively, ±400 mV. Note that a -2-V common-mode signal limits the in-amp's output to ±4V. The combined characteristics of all the in-amp's internal circuits define the envelope.
Texas Instruments provides a free Windows-based envelope-plot program for 17 types of its in-amps. No documentation accompanies this software, though. (See references, below.)
2. Don't let in-amp inputs float. You can aim to eliminate common-mode signal before they reach an in-amp, but watch your step. If you have an ac sensor signal, for example, you might try to use capacitors on in-amp inputs to block dc common-mode signals. But that circuit includes no return path for bias currents on the in-amp's inputs, which can float and "saturate" the output at either the plus or minus supply voltage. (I encountered this problem early in my career and it took a while to figure it out.) In-amp inputs always need a bias-current path to ground. A high-resistances (100-Mohm) from each in-amp input to ground provides the needed ground-return path, but does not appreciably alter the amplifier's capabilities.
If your sensor (thermocouple, capacitive sensor, transformer) circuit does not include a high-resistance dc path to ground, you must include one. An in-amp in data-acquisition equipment might already provide such a path. Check instruction manuals before you add external resistors.
This plot shows the operating envelope for an IN118 in-amp's output voltage for a given set of gain, common-mode voltage and power-supply conditions.
1. "Calculate Input Common-Mode Range of Instrumentation Amplifiers," (INA-CMV-CALC), Texas Instruments: http://designnews.hotims.com/23122-523
