Filter DC voltages outside your supply rails

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. This article, by 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.  [Note: you can see all of his Filter Wizard pieces here.]

The background: 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. In “Lowpass filters That Don’t, we saw an approach that can mitigate this to some extent. In the final circuit presented there (see Figure) 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).

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 5V 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 as the input – just cleaner.

To read this latest installment from the Filter Wizard, click here.

About the author
Kendall Castor-Perry is a Principal Architect at Cypress Semiconductor Corp., doing mixed-signal system analysis and design for the new PSoC platform.  Kendall uses decades of experience in analog engineering, filtering and signal processing to capture signals across many domains, extract the information from them and do something useful with it.