AN1636 Datasheet, PDF(34/42 Page) STMicroelectronics – UNDERSTANDING AND MINIMISING ADC CONVERSION ERRORS

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AN1636 Datasheet, PDF (34/42 Pages) STMicroelectronics – UNDERSTANDING AND MINIMISING ADC CONVERSION ERRORS

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UNDERSTANDING AND MINIMISING ADC CONVERSION ERRORS

Using error parameters

1. The TUE is NOT the sum of all the errors EO, EG, EL, ED. It is the maximum error which can

occur between ideal and expected digital values. It can be the effect of either a single error

or two errors occuring simultaneously.

2. As ILE is the integral of DLE, it can be considered as indicative of maximum error. Do not

add both DLE and ILE together to calculate the maximum error which can occur at any dig-

ital step.

3. Integral Linearity Error is the maximum deviation between any actual transition and the end

point correlation line. So it represents the linearity of the ADC.

4. ILE and DLE are dependent on the ADC design. It is difficult to calibrate them.

5. The ILE and DLE can be minimized by doing multiple conversions and then averaging.

6. Offset and Gain errors can be easily cancelled / compensated using software techniques.

7. The maximum values for errors specified in datasheet are the worst error values measured

in the laboratory test environment for full voltage range.

8. As already mentioned, all the ADCs provide the digital converted output in ratio with the ref-

erence voltage. To convert analog voltage accurately, the ADC needs to have an accurate

VAREF otherwise the digital output received may not be the correct value.

Example:

In the widely used 7805 voltage regulator, the datasheet (L78M05C) specifies Vo min.= 4.8V

and Vomax = 5.2v. This variation is 0.2V from required 5V. Hence a variation of 200mv. With

an analog input of 1V and with VAREF=5V, the expected digital output should be (1/5)*1023 =

204d = CCh

But if VAREF = 4.8v, the digital output = (1/4.8)*1023 = D5h

So the power supply variation has caused a change of D5-CC = 9.

In this case to measure the true input voltage, all the relevant parameters must be verified and

crosschecked. For example, if a precision voltage regulator such as the L78M05AB is used, it

provides Io=350mA and Vomin=4.9V, Vo max = 5.1V.

6 PCB LAYOUT RECOMMENDATIONS

For best ADC conversion accuracy, you should follow these PCB layout guidelines.

1. Separate the analog and digital layouts

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