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John Caldwell

In this month’s post on our sister blog, the “Precision Hub” on Planet Analog, I share a recent question I got from a customer in the Precision Amplifier Forum here in the TI E2E™ Community.

His circuit used an op amp to amplify the output of a microphone at very low frequencies. He used a large (47uF) AC coupling capacitor and a high input resistance (100kΩ) to achieve a low corner frequency for his measurements.

Unfortunately, a significant amount of DC offset was appearing at the output of the op amp – almost a volt!

What could cause this?

Check out my full post, “Plug that leak: Look out for capacitor leakage!,” on Planet Analog to find out.

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  • Hi Neil,

    All great points, the customer was already using a FET input amplifier (TL082) and reducing the capacitance value to allow for the use of different dielectric types was the eventual solution that I recommended.

    On the contrary, because the voltage drop across AC coupling capacitors is low within the desired bandwidth, the distortion electrolytic capacitors contribute is low when used in this way. I have also written about these effects in ceramic capacitors with high-k dielectrics (www.edn.com/.../Signal-distortion-from-high-K-ceramic-capacitors) In any case, capacitor distortion is proportional to the voltage across the capacitor. I agree that electrolytic capacitors would contribute distortion in active filter circuits, and at frequencies below the desired cutoff in coupling applications.

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  • Hi Neil,

    All great points, the customer was already using a FET input amplifier (TL082) and reducing the capacitance value to allow for the use of different dielectric types was the eventual solution that I recommended.

    On the contrary, because the voltage drop across AC coupling capacitors is low within the desired bandwidth, the distortion electrolytic capacitors contribute is low when used in this way. I have also written about these effects in ceramic capacitors with high-k dielectrics (www.edn.com/.../Signal-distortion-from-high-K-ceramic-capacitors) In any case, capacitor distortion is proportional to the voltage across the capacitor. I agree that electrolytic capacitors would contribute distortion in active filter circuits, and at frequencies below the desired cutoff in coupling applications.

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