Column: Circuit drill
The DC offset of the Sallen-Key low-pass filter
By Sulaiman Algharbi Alsayed, Managing Director, Smart PCB Solutions T
he Sallen-Key low-pass filter is one of the most popular second-order active low-pass filters, with a design similar to that of a voltage-controlled voltage
source (VCVS), with filter characteristics that include high input impedance, low output impedance and good stability. The circuit consists of one op-amp,
a couple of resistors and capacitors; see Figure 1. Despite its simple design, its efficiency is quite high, with a sharp drop in output signal amplitude when a high- frequency signal is applied to the input. At first glance its frequency
performance appears smooth, but we must check if there are any DC-offset components at the output that need to be compensated for with a DC-offset circuit. This is an essential thing any circuit designer should verify before finalising a design.
Experiment setup We connect the low-pass filter to a function generator that generates a sinusoidal signal with frequencies from 1Hz to 100kHz (no need to go any higher for this circuit). The input signal’s frequency was increased gradually, at fixed intervals. We also assumed that: • The input signal amplitude was kept constant at 5Vac throughout the experiment;
• R1, R2, C1, C2 and the op-amp were kept unchanged;
• The power supply voltage was kept constant to avoid interfering with the collected data;
• The circuit output was not connected to any load.
The DC-offset voltage was measured and logged at each input signal frequency. After a few milliseconds, when the signal
stabilises, we measure the DC- offset voltage, which is then plotted against the varying input signal frequencies; see Figure 2. The curve shows that the
Sallen-Key low-pass filter circuit generates a DC-offset component at the output that can be damaging to any circuit connected to the filter. In Figure 3, we see that the
output signal DC-offset voltage is on the negative side for input frequencies below 27kHz, while it is positive when input frequencies are above that limit. This is an important finding and must be considered by circuit designers, since they must add a DC-offset circuit at the output of this filter to eliminate any potentially damaging DC-offset voltage components.
Figure 1: Sallen-Key low-pass filter 08 March 2023
www.electronicsworld.co.uk
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