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Measuring 10mV analogue signal


polus
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You could make your own ADC with comparators. The more you string in line, the more resolution you will get. Well, up to a limit, of course.
But then, since you are familiar with micros, why not just choose one which has a built-in ADC? 10 bit ADC is pretty common.

MP

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A 10mV signal is small, but not the smallest. After you amplify, you must know that what you are getting is multiplied representation. The gain can be difficult to accurately guess. So when you finally divide the result to find the actual value you will have errors in your findings. This is the problem with amplification, you can't be exactly sure of the original.

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Kevin, if he uses a micro with a 10 bit ADC, he will not have to divide the signal back down again. For example, he could amplify the signal to 1 volt, feed it into the ADC of the micro, and the numeric representation would be what he would use. This could then be formatted and sent to a display as xx.x volts, etc.

MP

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I was thinking of using a 16-bit ADC on an analogue devices 8051 (I have a couple of samples... :) )

Anyway for the application accurately amplifying this sort of signal what kind of amplifier would you guys recommend?

To be honest one of the reason Im doing this is I really want to know more about analogue electronics and expand my digital knowledge because Im a software nut and I enjoy messing around with electronics. Ive done all sorts of high level programming in C++ and some embedded in C but my knowledge really falls short at the electronic design level (something Im hoping to change).

Thanks for your ideas, even the simplest of advice helps.

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The first question is what is the frequency of the signal you are measuring? How fast is it typically changing? You need to know this because you will have to handle it with filtering otherwise you will get aliasing when you sample it. The second question is how fast do you want to sample it? Do you want a plot of the signal or simply a digital readout of the signal that changes every second or so? Is your input signal bipolar (positive and negative)?

To amplify the signal you should use an instrumentation amplifier. You can get them contained in an IC or you can make one yourself out of operational amplifiers (opamps). It consists of a standard difference amplifier buffered at the front end by two more opamps. You might want to make your own if you are looking to learn analog electronics. Make sure the bandwidth of the opamps are sufficiently high.

You should select the gain of the amplifier to take advantage of the full swing of your ADC. For example, if the maximum input signal is 10 mV and your ADC range is from 0 to 10 V, you want a gain of around 1000. That way you won't have very large quantization noise.

Once you have the amplification, you need to address the questions I brought up in paragraph 1. I'll be glad to help you answer them once I know more detail of the input signal.

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