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Topic: Use of op-amp to sample AC signal with single-ended ADC (Read 955 times) previous topic - next topic

Constantin

Nov 15, 2012, 03:19 pm Last Edit: Nov 15, 2012, 03:25 pm by Constantin Reason: 1
Hi everyone,

I am currently in the process of trying to implement a ADE7753 in a single-phase energy meter. However, as a fallback, I am also considering using a teensy 3.0 because of its fast quasi-13 bit adc.

Unlike the ADE7753, the teensy uses a single-ended adc whose input is positive-only. I could follow in the footsteps of the energy monitor project as I have in the past ie have a strong dc bias get wobbled by the attenuated transformer output.

But I also ran across differentiating op amps as another potential solution to turn an AC signal from the transformer (+\- 5V) into a DC-biased signal suitable for the teensy, ie 1.65V +\- 1.5V since the teensy is limited to 3.3v on its adc inputs.

What are your thoughts on using an op-amp for this sort of application? Is it overkill when sampling line frequencies?  

As a second question, when it comes to monitoring the output of a transformer, should I built a voltage  divider using resistors that results in the right input range for the op amp, or is it better to choose among the few differentiating op amps that are designed for attenuation to do the same?

Many thanks for all the insights you can share, Constantin

dhenry

Even if you use an avr with differential adc inputs, each input has to be above ground - there is no way around that.

There are a lot of ways to measure ac signals (voltage or current). The typical approach would be to directly adc an ac signal with a dc offset (the one you are using). You could also adc the ac signal directly - knowing that in the negative half the signal is clamped / you read 0.

You can also rectify the ac signal before adc'ng it. etc. - that approach has its own issues.

For metering purposes, however, I would suggest that you look into dedicated chips or some form of multiplier - this works well in cases where your load isn't entirely / largely resistive.

dc42

I can't see any purpose in using an op-amp in this application, because you don't need amplification and you can generate the DC bias more easily using 2 resistors. A voltage divider is also the simplest way to reduce the signal to the required level.
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