Instrumentation amplifier datasheets typically show a graph (or several variations) of the Output Swing vs Input Common-Mode Voltage, also known as the Diamond Plot, which is a comprehensive graph of all external and internal headroom limits. Given enough basic information about a circuit, the Diamond Plot can be adjusted for various Supply Voltages, Gains, and Reference Pin Voltages
Although this adjusted plot itself is good information, the real value is in determining whether or not the signal of interest is within the Diamond Plot limits. This requires having the input signal information as well. The input signal range is plotted directly on the Diamond Plot and any errors where the input signal crosses the Diamond Plot boundaries are shown on the plot and are reported by the tool.
For any circuit configuration, the tool keeps a list of Recommended In-Amps. These Recommended parts meet the Diamond Plot requirements as well as Supply Voltage Range, REF pin voltage range, and Gain Range, so that they will work in the circuit as-described. This list can be filtered further within the tool or in the parametric table on analog.com. Using this tool during a design, in-amps can be selected within minutes, not hours, and with a high degree of confidence.
Finally, a simplified version of the Internal Circuitry is available for those who are curious about the inner workings of the in-amps and in finding specifically what is saturated. Sliders are available to adjust the voltage displayed across the entered signal range.
: In the Circuit Configuration Panel, choose a product from the drop-down list and enter the circuit conditions, such as supply voltage, reference pin voltage, and gain. Enter the input signal range, including any expected variations due to component tolerance and common-mode or differential offsets that must be accounted for in the design.
: Input common mode voltage is defined as the average of the two input voltages: VCM = (V+IN + V-IN) / 2. Therefore single-ended input signals, in which the voltage at one input is fixed and the other is varied, will change the common-mode voltage along with the differential voltage. In the drop-down, choose whether the input signal is fully differential or single-ended. And if single ended, is it the positive input which is varying, for example, a thermocouple with the negative lead grounded, or is it the negative input which varies, for example, sensing high-side current on a smaller voltage rail.
: If there are errors in the circuit, the tool shows them. There may be alternative products which do work, which are shown in the Recommended tab and also visible in the product selection drop-down menu. Otherwise, adjust the circuit until you arrive at a working configuration.
If selecting a product, use the Recommended tab to filter and sort the Recommended list. Access the product pages and datasheets through the provided links.
If using the Internal Circuitry tab, move the sliders to adjust the common-mode and differential voltage of the display point to any voltage in the Input Signal range.