SBOA508 January   2022 INA818 , INA819 , INA823 , INA849 , OPA2392 , OPA391 , OPA392 , OPA396 , OPA397 , OPA3S328

 

  1.   Trademarks
  2. 1Single N-FET or P-FET vs Complimentary N-P-FET Input Stage Limitations
  3. 2Example 1: Output Swing Limitation from VCM in an Op-Amp for Low Side Current Sense
  4. 3Bipolar and CMOS Output Stage Topologies and Output Swing Limitations
  5. 4Example 2: Output Swing Limitations With Instrumentation Amplifiers
  6. 5Summary
  7. 6References

4 Example 2: Output Swing Limitations With Instrumentation Amplifiers

Let’s go back to our current shunt example, but this time the INA326 device is used in a gain of 100 V/V, and we power with a 5-V supply. Let's say we want to measure current between 1 A and 50 A, and we choose a 1mΩ shunt resistor. This means we will see a minimum of 1 A × 1 mΩ × 100 V/V = 100 mV on the output. The swing-to-GND of the INA326 is -VS + 0.02 V, so an expected minimum output of 100 mV is acceptable. However, at 50 A current, the output voltage would be 50 A × 1mΩ × 100V/V = 5 V. The swing to rail spec is +VS – 75mV, worst case, and 5 V exceeds that.

So, what we can do to improve the circuit?

  1. Increase supply by at least 75 mV over 5 V (thus, increasing the output swing of the amplifier). This isn't usually an option in designs as power supplies are generally fixed at common values like 1.8 V, 2.5 V, 3.3 V, 5 V, and so forth. Also, the variation (minimum) of the supply needs to be considered, and has to always exceed 5.075 V to ensure linear operation.
  2. Lower the gain. Setting a lower gain value would result in a lower maximum expected output voltage, keeping the amplifier within the linear mode of operation, at the expense of lower measurement resolution.
  3. Choose a smaller shunt resistor. Reducing the value of the shunt resistor will decrease input signal and subsequently the output signal. Switching from 1 mΩ to 0.5 mΩ would keep the output well within the permissible range, at the expense of measurement resolution.
  4. Choose a different amplifier with more suitable supply voltage specifications. For instance, the INA823 has a swing-to-positive supply spec of +VS – 150 mV, thus switching from the INA326 to the INA823 by itself would not resolve the output swing issue. However, the INA823 can support power supply voltages up to 36 V, so if the supply voltage was increased to the next higher available rail, this would ensure output remains within the linear range of the amplifier (this solution is applicable to op-amps as well).