Design Goals

Design Description

This circuit produces an AC-coupled output from a DC-coupled input to an instrumentation amplifier. The output is fed back through an integrator, and the output of the integrator is used to modulate the reference voltage of the amplifier. This creates a high-pass filter and effectively cancels the output offset. This circuit avoids the need for large capacitors and resistors on the input, which can significantly degrade CMRR due to component mismatch.

Design Notes

1. The DC correction from output to reference is unity-gain. U₁ can only correct for a signal within its input/output limitations, thus the magnitude of DC voltage that can be corrected for degrade with increasing instrumentation amplifier gain. See the table in Design Steps for more information.
2. Large values of R₁ and C₁ lower the cutoff frequency, but increase start up transient response time. Start up behavior is observed in the Transient Simulation Results.
3. When AC-coupling this way, the total input voltage must remain within the common-mode input range of the instrumentation amplifier.

Design Steps

1. Set the lower cutoff frequency for circuit (integrator cutoff frequency). The upper cutoff frequency is dictated by the gain and instrumentation amplifier bandwidth.
   ${\mathrm{f}}_{\mathrm{L}}=\frac{1}{2\mathrm{\pi }\times {\mathrm{R}}_1\times {\mathrm{C}}_1}=\text{16 }\mathrm{Hz}$
2. Choose a standard value for R₁ and C₁.
   $\begin{array}{l}{\mathrm{C}}_1 = \text{100 }\mathrm{nF}\\ {\mathrm{R}}_1=\frac{1}{2\mathrm{\pi }\times \text{100 }\mathrm{nF}\times \text{16 }\mathrm{Hz}}=\text{99.47 }\mathrm{k\Omega }\approx \text{100 }\mathrm{k\Omega } \left(\mathrm{standard} \mathrm{value}\right)\end{array}$
3. The DC rejection capabilities of the circuit degrade with gain. The following table provides a good estimate of the DC correction range for higher gains.

   Gain	DC Correction Range
   1V/V	±10V
   10V/V	±1V
   100V/V	±0.1V
   1000V/V	±0.01V

Design Simulations

AC Simulation Results

Transient Simulation Results

Design References

Texas Instruments, SBOMAU0 TINA-TI™ circuit simulation, file download

Texas Instruments, TIPD191 Instrumentation Amplifier with DC Rejection, reference design

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