SBOA503 July   2021 INA101 , INA103 , INA111 , INA114 , INA115 , INA118 , INA121 , INA122 , INA125 , INA126 , INA128 , INA128-HT , INA129 , INA129-EP , INA129-HT , INA141 , INA155 , INA156 , INA1620 , INA163 , INA1650 , INA166 , INA188 , INA2126 , INA2128 , INA2141 , INA217 , INA2321 , INA2331 , INA2332 , INA317 , INA321 , INA322 , INA326 , INA327 , INA330 , INA331 , INA332 , INA333 , INA333-HT , INA333-Q1 , INA337 , INA338 , INA818 , INA819 , INA821 , INA823 , INA826 , INA826S , INA827 , INA828 , INA848 , INA849

 

  1.   Trademarks
  2. 1Typical Instrumentation Amplifier Operation
  3. 2Problem Statement
  4. 3Common Mistake
  5. 4Proposed Solution
  6. 5Component Selection and Trade-Offs
  7. 6Common Applications
  8. 7Conclusion

2 Problem Statement

In many IA applications, accurate, low-level ac signal processing is desired, while dc signals are rejected at the output. The easiest way to implement these functions is to ac-couple the IA. Designers ac-couple by adding capacitors in series with the IA inputs to block the dc input voltages, which effectively forms a high-pass filter. This method eliminates the need to accommodate the dc input signal before the IA gain stage pushes the dc input signal to saturation, a nonlinear operating condition. Therefore, this method of passing only the ac signal allows for higher gain and wider dynamic range.

For example, assume an IA has a 100-Hz sine wave with an amplitude of 100 mV in the presence of a 5-V common-mode voltage and a 3-V dc voltage. The desired output is a ±1-V signal. Using these operating conditions, the instrumentation amplifier must be configured with a gain of 10V/V. Applying the INA818, one of TI’s high-precision, low-power, low-noise IAs, the circuit schematic and transient analysis is shown in Figure 2-1.

Figure 2-1 INA818 Schematic and Transient Analysis

While the 5-V common-mode voltage is rejected by the IA, the 3-V dc voltage sums with the input differential voltage shown by the VDiff curve. In a gain of 10V/V, the output signal is saturated against the positive power supply rail. Even though the desired signal to amplify was the 100 mV / 100 Hz sine wave, the 3-V dc voltage prevented the instrumentation amplifier output from representing only the amplified ac signal.