Problem with reduced voltage after a LPF is added

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ahdavewest751

New Member

• Feb 9, 2012

• #1

Hello, I have a problem with my circuit. I have a converted 5V AC voltage to a 5V DC voltage and the output is put through a LPF with 100k Resistor and 1u Capacitor to reduce the noise but when a Howland voltage to current circuit is attached after the LPF the voltage is reduced to 460mV.

Can anyone offer any advice why this is happening?

 

Nigel Goodwin

Super Moderator

Most Helpful Member

• Feb 9, 2012

• #2

Fairly obviously 100K is MUCH too high a resistor, what is the input impedance of the load?.

 

audioguru

Well-Known Member

Most Helpful Member

• Feb 9, 2012

• #3

If the 100k resistor is reduced to 10k ohms and the 1uf capacitor is increased to 10uF then the DC voltage will be higher and the filtering will be the same.
Also try 1k ohms with 100uF or 100 ohms with 1000uF.

 

ronsimpson

Well-Known Member

Most Helpful Member

• Feb 9, 2012

• #4

What frequency of the PWM running at?
What ripple voltage can you withstand?

 

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ahdavewest751

New Member

• Feb 9, 2012

• #5

Reducing the resistance and cap has done the trick. Ideally I wanted to use phase sensitive detection to take an AC input from an amplifier and mix it with a ref signal to produce a rectified output with noise removed but I have no idea how to do this. Anyone no how to do this?

 

audioguru

Well-Known Member

Most Helpful Member

• Feb 9, 2012

• #6

Noise might be at all frequencies, not just high frequencies. If you reduce the noise then you also reduce voice and music frequencies.
It might be better to prevent the noise. Is it coming from a lousy old opamp? Then use a modern low-noise audio opamp instead.

 

crutschow

Well-Known Member

Most Helpful Member

• Feb 10, 2012

• #7

Post deleted

 

Last edited: Feb 10, 2012