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Signal Chain Basics #78: How to Avoid Common-Mode Limitations on Instrumentation Amplifiers

Instrumentation amplifiers (INAs) are used to amplify small differential voltages that are superimposed on dc common-mode voltages.1 A typical example is a bridge sensor output voltage where the differential sensor output is in the millivolts, and the large dc common-mode voltage is at midsupply voltage (VCM = 0.5 X VS ). Many types of INAs are optimized for common-mode signals at midsupply voltage. Instrumentation amplifiers optimized for midsupply common mode are useful in many cases, but they can have limitations for signals with different common-mode levels. This article explains common-mode limitations and how to achieve good common-mode range.

INA common-mode limitation
Figure 1 shows a common-mode voltage vs. input voltage plot taken from a data sheet for a precision instrumentation amplifier — in this case, the INA826. The plot shows the output voltage swing for two different reference voltages, 5V power supply, and gain equal to 100. The horizontal green line represents the circuit's common-mode range (Figure 2). Note that the output swing is optimized with the reference and common mode set at midsupply (0.1V < VOUT < 4.8V).

Figure 1

A typical output voltage limitation, in this case for the INA826; VS = 5V, VREF = 2.5V, G = 100.

A typical output voltage limitation, in this case for the INA826; VS = 5V, VREF = 2.5V, G = 100.

Figure 2 shows a typical INA application, where the common mode is at midsupply, and the differential input is in millivolts. Also, note that the reference pin is driven to midsupply. It is important that the reference is buffered to avoid loading of the voltage divider.2 This circuit's output is the differential input multiplied by the gain (VOUT = 20mV X 100 = 2V).

Figure 2

INA826 bridge sensor amplifier, VCM = 1/2 X VS.

INA826 bridge sensor amplifier, VCM = 1/2 X VS .

The green line in Figure 3 shows the output swing limitations for the circuit shown in Figure 4. In this case, the common-mode voltage is approximately zero; this limits the output range to a maximum of approximately 1.5V.

Figure 3

Output voltage limitations for INA826, VS = 5V, VREF = 0V, G = 100.

Output voltage limitations for INA826, VS = 5V, VREF = 0V, G = 100.

Figure 4 shows the INA826 being used as a current shunt monitor. In this case, the common-mode voltage is approximately zero. The ideal output for this circuit is 2V (VOUT = 100 X 20mV). Here, the output cannot achieve the expected value, because it is outside the output voltage range (0.1 < VOUT < 1.5V from Figure 3). However, the INA326 shown in Figure 5, with its less restrictive common-mode and output swing limitations, is a better choice for this application.

Figure 4

Current shunt amplifier based on the INA826, VCM = 0V.

Current shunt amplifier based on the INA826, VCM = 0V.

Special INA common-mode limitation
The circuit in Figure 4 had common-mode limitations that were inherent to the amplifier. Figure 5 shows the same circuit using a unique INA that avoids common-mode limitations. The INA326 uses a unique current mirror topology to amplify differential signals and reject common-mode signals. This unique topology has an advantage over most INAs, because it can amplify signals with common-mode voltages just beyond the power supply rails, as shown in Table 1 (-0.02V < VCM < 5.1V). Furthermore, the output swing range is very close to the power supply rails and does not depend on the input common-mode signal (0.005V < VOUT < 4.995V). Thus, the new approach is a good choice when you need to amplify signals that have common-mode voltages that approach the power supply rails.

Figure 5

Current shunt amplifier based on the INA326, VCM = 0V.

Current shunt amplifier based on the INA326, VCM = 0V.

Table 1

Information extracted from the data sheet showing common-mode range and outputswing, based on the INA326.

Information extracted from the data sheet showing common-mode range and output
swing, based on the INA326.

Please join us next time, when we will discuss digital temp sensors versus thermistors.

References:

  1. Semig, P., & Wells, C., EE Times, Current Sensing Fundamentals Part 1, Feb. 2012.
  2. Hann, G., Electronic Products, November 2008.
  3. For more information visit, www.ti.com/amplifiersandlinear-ca.

— Arthur Kay is an applications engineering manager at TI, where he specializes in the support of amplifiers, references, and mixed-signal devices. He focuses a good deal on industrial applications such as bridge sensor signal conditioning. He has published a book and an article series on amplifier noise. He received his MSEE from Georgia Institute of Technology and his BSEE from Cleveland State University. He can be reached at scb@list.ti.com.

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