Datasheet

ADC0844, ADC0848
www.ti.com 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
• f
CM
is the frequency of the common-mode signal
• V
peak
is its peak voltage value
• t
C
is the conversion time
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.
Table 2. ADC0848 MUX Addressing
(1)
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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