Author Topic: Amplify and offset AC signal  (Read 2279 times)

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Offline LukeWTopic starter

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Amplify and offset AC signal
« on: April 19, 2014, 06:10:02 pm »
Suppose you've got a small AC signal centred around 0V, and you want to amplify this signal a bit whilst also introducing a DC offset of +Vcc/2.

The goal is to amplify and shift up the signal to Vcc/2 so that the entire waveform is always positive and in the 0-Vcc range so it can be sampled by a single-ended ADC, so that the minima of the signal is just above 0V and the maxima of the signal is just below Vcc, where Vcc is the voltage reference that will be used with the ADC, without using any negative power supplies for the opamp, using the simplest minimalist circuit.

I've attached an example simple schematic as a starting point... but how would you do this differently, or improve it?
 

Offline miceuz

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Re: Amplify and offset AC signal
« Reply #1 on: April 19, 2014, 06:39:04 pm »
I'd buffer R5/R6 with another opamp. Then probably you will want to add a trimpot somewhere to trim out offset errors of opamps and resistor tolerances. And lastly your amplified signal does not fit into 3.3V.

165e-3*10.1 + 3.3/2 = 3.315V and the opamp surely will never rail to 3.3V. You have to lower the gain.

Offline LukeWTopic starter

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Re: Amplify and offset AC signal
« Reply #2 on: April 19, 2014, 07:12:31 pm »
Yeah, close enough. It was just a roughly guessed example value to illustrate the concept in question, obviously the gain can be fine tuned. And let's assume the opamp is rail-to-rail. :)
 

Offline pmbrunelle

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Re: Amplify and offset AC signal
« Reply #3 on: April 19, 2014, 07:24:33 pm »
I'd buffer R5/R6 with another opamp. Then probably you will want to add a trimpot somewhere to trim out offset errors of opamps and resistor tolerances. And lastly your amplified signal does not fit into 3.3V.

165e-3*10.1 + 3.3/2 = 3.315V and the opamp surely will never rail to 3.3V. You have to lower the gain.

No point in buffering the R5/R6 divider if you're just going to tack on a 10k series resistance. Instead, replace R4 with a short, and increase R5 and R6 to 20k.
 

Offline dannyf

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Re: Amplify and offset AC signal
« Reply #4 on: April 20, 2014, 02:09:56 am »
Quote
but how would you do this differently, or improve it?

Replace R5/R6 with a capacitor and call it a day.
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