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PIC a to d conversion sample rate

:: EPE Chat Zone ­:: ­Radio Bygones Message Board :: » EPE Forum Archives 2007-2009 » Archive through 27 May, 2008 » Interested in making money?? » PIC a to d conversion sample rate « Previous Next »

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tommy
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Username: tommy

Post Number: 10
Registered: 10-2007

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Posted on Tuesday, 06 May, 2008 - 08:54 pm:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

I am working on a small project to build a current meter using a current transformer, AC to DC RMS conversion chip, an opamp, a PIC16F690 and a character display.

Whilst writing the software I used a 5K potentionmeter to vary the voltage on the a to d pin between 0 - 5V. I had no problems calculting and displaying the voltage level on the display in increments of 0.01V.

I have now built the rest of the circuit and to test the meter I linked the current transformer upto the feed to a 400W heater. The output of the opamp when measured with a multi meter shows 1.7V exaclty the same value as the 1.7A drawn by the heater measured on my clamp meter.

Now here is the problem: When I link the output of the opamp directly to the a to d pin on the PIC. The value on the display flickers and all 3 numbers change very rapidly and cannot be read.

My assumption is that the output impedance of the op amp is lower than impedenace of the 5K pot. In this case, the internal capacitor of the a to d circui inside the pic charges quicker, thus speeding up the conversion. Is this assumption correct?

To correct the problem, is this sinmply a case, of putting a longer delay in the software between completing each sample and writing the result to the display.

My present settings in the software are:

fosc 8
50ms delay between samples and display

Is it better to have a longer sample time dbetween samples or is there some disadvantage to this.

Sorry for such a long question.
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magnum4
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Username: magnum4

Post Number: 595
Registered: 04-2005


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Posted on Tuesday, 06 May, 2008 - 09:17 pm:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

If its a AC load, the current will be changing rapidly. This probably can be seen on a scope.

I would try a small cap. 1uF maybe on the adc input pin, This is to smooth the input.

Also its a good idea to take maybe 10 readings and then only display the average.


HTH
Regards,
Jim

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