Column: Circuit drill
OPA376 AC coupler circuit DC off set and output load restrictions
By Sulaiman Algharbi Alsayed, Managing Director, Smart PCB Solutions Company
D
ue to the ubiquity of alternating current (AC) signals in many electronic circuits, designers have always faced the problem of protecting these signals
from intermittent DC off set voltages. T e AC signals generated in many
electronic circuits are not always guaranteed pure and free from any unwanted DC off set voltages. It is always important to maintain AC signals’ purity when fed to the load or another circuit. T erefore, many electronic circuits called “AC couplers” were created to knock out any unwanted DC off set voltages from the AC signals; see Figure 1. A well-known AC coupler circuit is
shown in Figure 2; it’s the subject of our experiment here. T is circuit uses the popular op-amp
IC OPA376 connected in a non-inverting confi guration, even though the circuit can also be connected in an inverting confi guration, where the output signal’s waveform is the inverted shape of the input signal. In the non-inverting confi guration, the output signal waveform is the same phase as the input signal. For simplicity, this experiment was
conducted using only the non-inverting confi guration; circuit behaviour was found the same in both confi gurations.
Why the experiment? The questions I had in mind are twofold: (1) Will circuit performance change at higher DC offset voltages? In other words: does the performance of the AC coupler circuit depend on how high the DC offset voltage is?
14 May 2021
www.electronicsworld.co.uk Figure 1: AC coupler circuit
(2) What is the lowest load resistance we can connect at the AC coupler output without affecting its performance? Since the manufacturer’s datasheet makes no mention, I decided to perform this experiment to find out. Typically, AC coupler circuits are
an integral part of a larger circuit, so knowing their limits is particularly important to avoid unwanted distortion in the valuable AC signals.
Methodology For the first part of the experiment, various DC offset components were injected accompanying the AC signal, and the output signal was monitored. In the second part, for the load resistance R5 (Figure 2), various values were applied, and the output signals monitored. I made the following assumptions: • The circuit is set for non-inverting
Figure 2: AC coupler circuit using OPA376
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