ADC0844, ADC0848

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SNAS523D – JUNE 1999 – REVISED MARCH 2013

THE ANALOG INPUTS

Analog Differential Voltage Inputs and Common-Mode Rejection

The differential input of these converters actually reduces the effects of common-mode input noise, a signal

common to both selected “+” and “

−” inputs for a conversion (60 Hz is most typical). The time interval between

sampling the “+” input and then the “

−” inputs is ½ of a clock period. The change in the common-mode voltage

during this short time interval can cause conversion errors. For a sinusoidal common-mode signal this error is:

where

•

•

•

For a 60 Hz common-mode signal to generate a ¼ LSB error (

≈5 mV) with the converter running at 40 μS, its

peak value would have to be 5.43V. This large a common-mode signal is much greater than that generally found

in a well designed data acquisition system.

MUX Address

Channel

CS

WR

RD

MUX Mode

MA4

MA3

MA2

MA1

MA0

CH1

CH2

CH3

CH4

CH5

CH6

CH7

CH8

AGND

X

L

L

L

L

L

H

+

−

X

L

L

L

H

L

H

−

+

X

L

L

H

L

L

H

+

−

X

L

L

H

H

L

NP

H

−

+

Differential

X

L

H

L

L

L

H

+

−

X

L

H

L

H

L

H

−

+

X

L

H

H

L

L

H

+

−

X

L

H

H

H

L

H

−

+

L

H

L

L

L

L

H

+

−

L

H

L

L

H

L

H

+

−

L

H

L

H

L

L

H

+

−

L

H

L

H

H

L

NP

H

+

−

Single-Ended

L

H

H

L

L

L

H

+

−

L

H

H

L

H

L

H

+

−

L

H

H

H

L

L

H

+

−

L

H

H

H

H

L

H

+

−

H

H

L

L

L

L

H

+

−

H

H

L

L

H

L

H

+

−

H

H

L

H

L

L

H

+

−

Pseudo-

H

H

L

H

H

L

NP

H

+

−

Differential

H

H

H

L

L

L

H

+

−

H

H

H

L

H

L

H

+

−

H

H

H

H

L

L

H

+

−

X

X

X

X

X

L

L

Previous Channel Configuration

(1)

X = don't care, NP = negative pulse

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