Editor’s Note: We welcome guest blogger Pete Semig, Analog Applications Engineer, Texas Instruments
The most prevalent TI E2E™ Community forum question concerning instrumentation amplifiers (IAs) revolves around common-mode voltage (Vcm) vs. output-voltage (Vout) plots. There are typically only a few of these plots in a data sheet based on commonly used power supply and reference voltage configurations. The issue is that the plots depend on the device’s supply voltage, gain and reference voltage. Therefore, if the final application uses power supplies, gain or a reference voltage that differ from those used to characterize the device, the plot will change.
In this post, I’ll use the INA333 TINA-TI™ Vcm vs. Vout model to show how the plot changes with supply and reference voltage for a traditional three-op-amp IA. For more detailed information about the theory, model and how the plot changes with gain, see the three-part article published on EDN, “Instrumentation amplifier VCM vs VOUT plots”: Part 1, Part 2 and Part 3 .
Figure 1 depicts a typical Vcm vs. Vout plot for the INA333. This plot shows operation given a dual supply of +/-2.5V and a mid-supply reference voltage (Vref = 0V). What happens to the plot if only +/ 1.8V supplies are available?
Figure 2 shows a TINA-TI schematic that uses the INA333 Vcm vs. Vout model given a dual +/-1.8V supply.
Figure 4 shows the result of simulating the circuit shown in Figure 2 with reference voltages of 0V and 1V. Increasing the reference voltage shifts the plot to the right, while decreasing the reference voltage shifts the plot to the left.
Finally, let’s look at single-supply operation. Figure 5 shows the effects of changing the reference voltage when operating the INA333 with a single 5V supply. Here, the plot shifts to the left as the reference voltage decreases from mid-supply (2.5V) to ground (0V).
Figure 6 shows the effects of decreasing the supply voltage while maintaining a mid-supply reference voltage. Both the common-mode range and output-voltage range decrease proportionally as the supply voltage decreases from 5V to 3.3V.
Notice that the plot shapes in Figure 6 are the same as those in Figure 3. Comparing the axes from +/ 2.5V dual-supply plot shown in Figure 3 with the +5V single-supply plot in Figure 6, the plot is now shifted up by 2.5V because both supply voltages increase by 2.5V. Additionally, the plot shifts to the right because the reference voltage is mid-supply (Vref=2.5V).
Thus, the Vcm vs. Vout plot for a traditional three-op-amp IA changes with supply voltage, reference voltage and gain. Decreasing the supply voltage decreases the common-mode and output-voltage ranges. Increasing the reference voltage shifts the plot to the right, while decreasing the reference voltage shifts the plot to the left.
Using a simulation model like TINA-TI, you can overcome the limitations of op amp datasheets when designing the right instrumentation amplifier for your system. Currently, TINA-TI Vcm vs. Vout models exist for the INA333 and INA826. These models can help you ensure that your instrumentation amplifier is operating in a linear region if your operating conditions differ from those shown in the data sheet.
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