Author Topic: Cap advice  (Read 8991 times)

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Offline IvoS

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Cap advice
« on: January 08, 2012, 06:10:19 pm »
I am building a small amplifier with first stage like this:

and have a question about the input cap. Is it important to have an input cap for this high pass filter with low leakage current? I want to use a cap 220-330uF with 100ohm resistor in series. The input voltage will be between 5-35VDC. The amp will measure DC noise.

Offline Conrad Hoffman

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Re: Cap advice
« Reply #1 on: January 08, 2012, 09:30:54 pm »
The leakage will be amplified by the circuit, but if the gain isn't too high you should be ok. I'd be more worried about the polarity of the cap matching the polarity of what's being measured. As long as you only measure positive or negative supplies, and have the cap installed accordingly it's ok, but one mistake...

Inverting circuits are also typically quite low impedance (100 ohms in your case) and will load whatever you're measuring. If the thing you're measuring isn't affected by an ac coupled 100 ohm load, great. I wouldn't take that as a given. People usually build a high impedance amplifier with a smaller cap value. That way you don't load the device under test, the cap can be a very high quality film cap, and leakage isn't an issue. In either case, you should also have protection on the input so excess voltage, transients and such don't damage the input stage of opamp.

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