AN2719 datasheet(10/22 Pages) STMICROELECTRONICS

10 / 22 page

ADC errors

AN2719

10/22

2.5

Hardware design errors

2.5.1

External resistance design error

The input multiplexer has nonzero impedance (Rmpx = max. 1kΩ). Sampling is done by

switch which has also nonzero impedance (Rsw = max. 30kΩ). Both these impedances,

together with the sampling capacitance (Csamp = max. 3pF) and external signal source

resistance (Rext), create a low pass filter. Therefore the external signal source impedance

(Rext) must be designed so that this low pass filter cut-off frequency will be not too low in

relation to the ADC sampling time (Ts = 3 ADC clocks).

Figure 10.

Analog input circuit

If the ADC converter has 10-bit precision (n=10) then the maximum precision is 1/2 LSB

level (0.5/1024 from full range). So the acceptable error caused by charging Csamp through

all resistors is 0.5/1024 (during 3 cycles of fADC clock). Then the maximum resistor value

Rmax=Rext+Rsw+Rmpx is obtained from following equation:

for worst case: fADC = 2 MHz and required 10-bit resolution (n = 10) the maximum serial

resistor is:

and the maximum external resistor is:

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 < 20 kΩ is suggested.

mpx

ext

() C

samp

⋅

STM8 ADC

SWsamp

Csamp

input

Rext

AIN

error

–

()

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

ADC

-----------

max

samp

⋅

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

–

⁄

----------

max

ADC

samp

⁄

----------

⎝⎠

⎛⎞

⋅⋅

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

–

max

210

⋅

() 310

–

⋅

()

⁄

1024

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

⎝⎠

⎛⎞

⋅⋅

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

–

66k

≅

ext

max

–

mpx

–

35k

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