Filter DC voltages outside your supply rails

March 17, 2011 // By Kendall Castor-Perry
Want to filter a bias, reference or even power supply voltage effectively, but only using circuitry that runs off a much lower supply rail? Might sound impossible, but it is not. Kendall Castor-Perry, the Filter Wizard, shows how it is done, with some theory in this article and a practical design in a forthcoming piece.

Active lowpass filters can have a wide range of AC response characteristics but they all have one thing in common, and that’s that they pass DC. Like other analog processing blocks, they can therefore introduce errors in both the DC offset and the gain or span.

Most filter configurations introduce DC offset error due to the offset voltages of the
amplifiers used. We saw, in “ Lowpass filters That Don’t ”, an approach that can mitigate this to some extent. In the final circuit presented there (shown again as Figure 1 below) the heavy lifting of frequency response management is done by an active “sidechain”, and the main signal just passes along a resistor network from input to output. The only contribution to offset error comes from amplifier input leakage currents dropping voltage across this resistor network. In modern MOS amplifiers these leakage currents are tiny (at least at room temperature).

(Click on image to enlarge)

Figure 1: A DC-coupled lowpass D-element filter with low offset voltage.

There are some use cases that this configuration still doesn’t support, though. One is where you can’t rely on the amplifier input currents being low enough to neglect. This might be the case, even with MOS amplifiers, if the filter is inside a seismic sensor sitting at the bottom of a very deep hole in the ground. The other case might not seem like a legitimate case at all at first glance. That’s where the voltage you want to filter is outside the supply voltages that you have available to run your active circuitry. For instance, say you’ve got a high-value bias voltage of tens or hundreds of volts, and you’re trying to lowpass-filter it to remove some ripple, but you only have a 5 volt power supply on your circuit board. Attenuating the voltage to ‘fit’ is out of the question, since you want the filtered voltage to be the same value

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