AN2353

Analog Digital Converter (ADC)

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Analog Digital Converter (ADC)

Analog input signal error can be created by poor matching of the source internal resistance

with the ADC input parameters, which can be caused by the following occurences:

●

Voltage drop in the voltage source resistance due to input leakage current

●

Analog input error can also be caused by noise from the analog input signal.

This section describes each of these causes.

Figure 6.

Source internal resistance errors

6.1

Voltage drop in the source resistance

The error generated by the voltage source internal resistance is:

input pin; this protection is necessary to protect the device against ESD (Electrical Static

Discharge) and against overload.

6.2

Poor charging of the ADC internal resistance

the external source. The internal resistance of the source must allow the capacitance to

reach its final value before the end of sample time.

If this does not happen, that is, if the source resistance is mismatched to the sample time, a

voltage loss occurs at the sample and hold stage. This voltage loss causes an accuracy loss

P5.x

R1

Sample

AGND

AGND

R1 (Series input resistance): maximum 1.5k

Ω (guaranteed by design characterization)

Leakage

Current

error LSB

()

R

SOURCE

I

0Z1

()

×

V

AREF

V

AGND

–

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

1024

×

=