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Old December 22nd, 2016, 02:35 PM   #1
wildcatherder
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Digital offset on analog input

On a Micrologix 1100 analog input, I appear to be getting an internal offset of about 11 counts out of 1023.
Running the program from RSLogix, when I look at N7:35, which is only written by the analog input, I see 6 counts out of 1023 when the analog input is shorted at the PLC input. If there is no applied input (and the short removed) I see 11 out of 1023 counts.
The measured input voltage in this case is .001 volts.

Does someone know where this offset comes from? It appears to be completely internal to the PLC.
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Old December 22nd, 2016, 02:56 PM   #2
g.mccormick
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Just offset/inaccuracy of the A/D in the card I'd assume.
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Old December 22nd, 2016, 02:58 PM   #3
wildcatherder
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Out of cal?

You would certainly think zero would be factory-calibrated.
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Old December 22nd, 2016, 03:02 PM   #4
g.mccormick
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What is the stated accuracy?
6/1023 * 100 = 0.5% of zero reading.

Is it really 1023, so 10 bit?
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Old December 22nd, 2016, 03:08 PM   #5
wildcatherder
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10-bits input

Overall accuracy is shown as +/- 1% full-scale, so that is 10 out 1023 and is good as it gets.

We were using the Micrologix 1000 with 16-bit input but switched because the 1000 is being discontinued.

Well, that's another mystery solved.

Thank you, G. McCormick for your assistance in this matter.

Last edited by wildcatherder; December 22nd, 2016 at 03:20 PM. Reason: added thanks
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Old December 22nd, 2016, 03:36 PM   #6
g.mccormick
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happy to help.

It's rare that I actually can help someone on here
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