AN1636 datasheet(21/42 Pages) STMICROELECTRONICS

Part #	AN1636

Description	UNDERSTANDING AND MINIMISING ADC CONVERSION ERRORS

PDF	42 Pages

Scroll/Zoom	100%

Manufacturer	STMICROELECTRONICS [STMicroelectronics]

Direct Link	http://www.st.com

Logo

21 / 42 page

21/42

UNDERSTANDING AND MINIMISING ADC CONVERSION ERRORS

Figure 17. ADC input with RAIN, CAIN

The external capacitance will not allow the voltage of the analog input to be exactly the same

as VIN if it is not fully charged by the analog source. If the analog input signal varies, then the

analog signal frequency (FAIN) should be such that the time period of this analog signal is at

least 10 * RAIN * CAIN.

Figure 18. Recommended RAIN, CAIN values

TAIN = time period of analog signal = 1/FAIN

TAIN >= 10 * RAIN * CAIN.

FAIN = 1/ TAIN <= 1/ (10 * RAIN * CAIN )

For example:

For RAIN = 10K, CAIN = 10nf

TAIN = 10 RAIN * CAIN

= 10 * 10K* 10n

= 1000µs = 1ms

FAIN = 1/TAIN = 1/1ms = 1KHz maximum.

AINx

VAIN

RAIN

CAIN

ST72x

Cp = Parasitic capacitance

CAIN = Source capacitance

0.1

100

1000

0.01

0.1

fAIN(KHz)

Cain 10 nF

Cain 22 nF

Cain 47 nF

AN1636 Datasheet(PDF) 21 Page - STMicroelectronics