AN3137 datasheet(19/42 Pages) STMICROELECTRONICS

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AN3137

ADC errors

Doc ID 16983 Rev 3

19/42

If the ADC converter has a 12-bit resolution (n=12) then the maximum precision is 1/2 LSB

level (0.5/4096 from full range). So the acceptable error caused by charging Csamp through

all resistors is 0.5/4096 (during tS sampling time = 4 to 384 cycles of fADC clock). Then the

maximum resistor value Rmax=Rext+Rsw+Rmpx is obtained from the following equation:

error

–

()

-----------------------

ADC

-----------

max

samp

⋅

------------------------------------

–

0.5

--------

max

ADC

samp

0.5

--------

⋅⋅

----------------------------------------------------------

–

For worst case: fADC = 16 MHz, 12-bit resolution (n = 12) required and sampling time = 4

clocks (tS = 4), the maximum serial resistor is:

max

⋅

()

–

⋅

()

0.5

4096

-------------

⋅⋅

-----------------------------------------------------------------------------------------------

–

1.7k

≅

and the maximum external resistor is:

ext

max

mpx

()

–

1.7k

Ω cca.1.2kΩ

()

–

500

≅

The external parasitic PCB and package capacitance must also be taken into account.

Therefore the maximum resistance of external signal source Rext is lower, and in practice

Rext < 500 Ω is suggested.

In case of longer sampling times and reduced number of ADC clocks, better results can be

obtained: for instance, sampling time can be extended to 384 cycles (instead of 4 cycles),

which offers an allowed resistance higher than 50 k

Ω (@ 16 MHz ADC clock). It is possible

to further increase the allowed external resistance by decreasing the ADC clock frequency

or selecting a lower resolution. Refer to the datasheet of your device to obtain the exact

values of the RC parameters.

3.5.2

ADC reference voltage source

One of the biggest potential sources of errors in an ADC is the reference voltage. It is

important to look at three reference voltage specifications: temperature drift, voltage noise,

long term stability.

3.5.3

Temperature influence

Temperature has a big effect on ADC precision. Mainly two specifications are important:

offset drift and gain drift. These errors can be compensated in microcontroller firmware. One

method is to fully characterize the offset and gain drift and provide a lookup table in memory

to correct measurement due to temperature change. The ADC in each MCU device must

then be compensated individually - therefore this calibration takes additional cost and time.

The second method is to recalibrate the ADC when the temperature change reaches given

values.

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