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removing offset and gain error of a ADC?

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vijay_nag

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how to overcome offset error in adc with software

hi all!

i would like to know how do we remove offset and gain errors of an ADC

regards,
vijay
 

cancel

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just do signal coditioning before ADC, shift and amplify/attenuate signal to fit in full span of ADC.
Results will be much better.
 

vijay_nag

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cancel said:
just do signal coditioning before ADC, shift and amplify/attenuate signal to fit in full span of ADC.
Results will be much better.

Hi cancel,

can you please explain in more detail?

regards,
vijay
 

sankudey

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Hi,

Its always better , if could be done, to identify why they are at all appearing and what is the exact source of the error. Then definitely u can remove it by taking care of the source of errors.

In general:
(1) Offset Error:
It may come from common mode mismatch and/or total voltage level mismatch of in intra/inter block levels of the ADC. To explain, say , there is a small voltage level shift between ur THA out put and next block. Suppose ur THA should give Vos ~ 1V to 2V for the proper input voltage. But for some reason it shifts from 1.1V to 2.1V. Then this 0.1V will appear as offset of the ADC , if not compensated after wards.

To take care of this thing, you may ADD/Subtract, i.e. , shift the ADC output voltage in digital domain. Or u may have tunable reference voltages such that this 0.1V could be adjusted. There may be some other options which the designer could say better. This is, once again, dependent on architecture and block level realisation and also circuits followed.


In simulation level: You may adjust as needed. AT post-layoyt simulation level u may have to back annote and correct it. After fabrication, its tough of some options are not kept open like said above or else.

(2) Gain Error:
Almost the same idea applies here. Say ur THA is giving a gin of 0.9 instead of 1.0. This will give a gian error of the ADC. U may do something like the above: Tunable reference points; gain adjustment option to the THA etc.

Please note that I have taken THA as the source of error for both of the cases above. It is not necessarily the case. So better to identify the source of the error. If it cannot be done; for gain error ; u may compensate with a block with gain ~ 1/(errored_gain) of the ADC before u feed it to the ADC.

sankudey
 

yok

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Friend, some ADCs came with offset error compansator that requires with only one external variable resistor for offset tuning.
 

lambtron

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how do we remove offset and gain errors of an ADC?
One way to do this is with an analog multiplexer and a voltage reference.

The mux is wired so it can connect 0V, Vref, or the external input voltage to the adc input under software control. First connect and measure 0V; the binary result is your offset reference value. Repeat for Vref; this is your full-scale reference value. Now connect and digitize the unknown external voltage; this value is mathematically corrected (in the digital domain) by standardizing it against the reference values.

The adc input span must be designed so that the worst-case offset and gain errors are guaranteed to be measurable. For example, say you want to measure 0 to +5V and your worst-case errors push this out to -0.5 to +5.5V. You would then design the adc to accept -0.5V to +5.5V range. Obviously, the adc must have a resolution that is slightly greater than your target resolution.

I have used this method in many designs and it works great! It will give you nice, stable data, even if you use cheap components that drift with time and temperature. Just be sure to use a good quality, low tempco voltage reference.
 

sankudey

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yok said:
Friend, some ADCs came with offset error compansator that requires with only one external variable resistor for offset tuning.

Hi,
It would be nice if u pls elaborate or give the mechanism or part number such that we can understand it better...

sankudey
 

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