# What is the best way to remove DC offset

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Could some electronics guru please help ? I am trying to remove a large (about 3.0V) DC offset from my signal. I have tried a capacitive divider and then AC coupling, but these are not very effective. Any hints, suggestions would be of immense help.

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A battery of two AA alkaline cells for 3 volts total.

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Transformer?```
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When posting a question, always answer the 'big six questions': *Who, What, Where, Why, When, How Much* You will only know which answers were important when your mystery is correctly solved.

You answered 1-1/2 questions of the six.

Who: Daku. Example: Tech in a college lab

What: Example: Interface between an 'optical color sensor front end' and a small 3 V microwave telemetry module with 120 ohm pullups in the transmitter

Signal with a troublesome DC offset. * Normal amplitude of the signal? Example: +- 50 mV p-p

• Type of signal? Example: Differential NRZ, 10 KHz

Where: Example: All on a wood lab bench isolated from other experiments

Why: Example: Plant biology experiment Recording leaf color changes

When: Example: Operated normally when I powered it from a lab supply for the 'color sensor' and two "D" cells for the transmitter, but +3 V DC offset appeared as soon as I removed the cells and plugged in a 'wall wart' to power the transmitter :)

How Much: Example: +3.0 V DC relative to earth ground

--Winston

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So, you are going to find a pair with exactly 3 volts total? Measure the terminal voltage of a new alkaline cell and you'll see that it's higher than 1.5 volts.

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You can't fix stupid. You can't even put a Band-Aid? on it, because it's
Teflon coated.```
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Please specify the type of data that is on this signal? Clean analog? Digital ? etc.. It makes a difference on the method used.

They make virtual grounds that can split the power supply so that your common is actually at center point. This means you need to have a isolated power source for this one circuit.

If you don't want to go that route, you can create a small (-) voltage source to network against the +DC offset you have via a divider.

But, like I said, with out knowing a little more of what you have, it's hard to select the proper way. Because I was, myself, employ a op-amp with minus offset uses as a buffer+amp if needed.. Hell, with rail To Rail types, one could use a dual type, one unit for the (-) volt generator and the other for the buffer+gain..

Jamie

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I once designed a transistor driver for a piezo strip using a couple batteries so the circuit could work off a single single hv supply.

Ac coupling needs grounding resistance after the cap.

Greg

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Or alternatively use a low pass filter to remove any fast signal variations and hence a DC reference is created. Using a differential amplifier across the LPF will show the fast variations (AC) only.

Not common in electronics design, but this approach is used much in image processing to remove illumination variations e.g. before edge detection.

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The circuit you describe is sounds like what is referred to as a "servo" input on high-end audio equipment. Such gear typically claims "DC coupled inputs" to alleve audiophile fears of musicality corruption from the dreaded input coupling capacitor in the signal path.

The idea is that the signal to be amplified does indeed pass through a DC-coupled stage, which also subtracts off a low-passed version of the signal from a parallel path. The LPF capacitor (typically in a "leaky integrator") is seeing the input signal, but essentially filtering it out completely to leave only DC (or maybe a fractional Hz, etc).

This allows AC coupling using a rather small and cheap capacitor... but I suspect the main advantage is that they can claim it as DC coupling. That removes any question that might have arisen about a conventional AC coupling circuit.

Otherwise, they'd need to use an exotic cap made out of passivated platinum titanate or something, and claim that it was better than their competitor's organically grown nanotubular zirconium capacitor.

Best regards,

Bob Masta DAQARTA v6.01 Data AcQuisition And Real-Time Analysis

Scope, Spectrum, Spectrogram, Sound Level Meter Frequency Counter, FREE Signal Generator Pitch Track, Pitch-to-MIDI Science with your sound card!

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I thought it was usually mermaid scales, liberally coated in snake oil with unicorn horn dielectric and gold wires these days!

Regards, Martin Brown

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```--
I thought mermaid scales were used to weigh pearls.```
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It certainly isn't swine like Pelosi, hanging them around her neck on a daily basis. I'll bet she isn't so happy any more.

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a

Who knows? She isn't Speaker of the US House of Representative any more, which means that she has less to do, but she is now the Minority Leader in the House, which is probably enough to keep her agreeably busy. She may be enjoying the less prominent - and presumably less taxing - role.

-- Bill Sloman, Nijmegen

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I didn't know mermaids even _had_ scales, being mammalian and all; more like a dolphin's tail - their flukes are horizontal, after all.

Cheers! Rich

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