Datasheet

ADC08831, ADC08832
SNAS015C –SEPTEMBER 1999–REVISED MARCH 2013 www.ti.com
FUNCTIONAL DESCRIPTION
MULTIPLEXER ADDRESSING
The design of these converters utilizes a comparator structure with built-in sample-and-hold which provides for a
differential analog input to be converted by a successive approximation routine.
The actual voltage converted is always the difference between an assigned “+” input terminal and a “−” input
terminal. The polarity of each input terminal of the pair indicates which line the converter expects to be the most
positive. If the assigned “+” input voltage is less than the “−” input voltage the converter responds with an all
zeros output code.
A unique input multiplexing scheme has been utilized to provide multiple analog channels with software-
configurable single-ended, or differential operation. The analog signal conditioning required in transducer-based
data acquisition systems is significantly simplified with this type of input flexibility. One converter package can
now handle ground referenced inputs, differential inputs, as well as signals with some arbitrary reference voltage.
A particular input configuration is assigned during the MUX addressing sequence, prior to the start of a
conversion. The MUX address selects which of the analog inputs are to be enabled and whether this input is
single-ended or differential. In addition to selecting differential mode the polarity may also be selected. Channel 0
may be selected as the positive input and channel 1 as the negative input or vice versa. This programmability is
illustrated by the MUX addressing codes for the ADC08832.
The MUX address is shifted into the converter via the DI line. Because the ADC08831 contains only one
differential input channel with a fixed polarity assignment, it does not require addressing.
Table 1. Multiplexer/Package Options
Part Number Number of Analog Channels Number of Package Pins
Single-Ended Differential
ADC08831 1 1 8 or 14
ADC08832 2 1 8 or 14
Table 2. MUX Addressing: ADC08832
Single-Ended MUX Mode
MUX Address Channel #
Start Bit SGL/DIF ODD/SIGN 0 1
1 1 0 +
1 1 1 +
Differential MUX Mode
MUX Address Channel #
Start Bit SGL/DIF ODD/SIGN 0 1
1 0 0 + −
1 0 1 − +
Since the input configuration is under software control, it can be modified as required before each conversion. A
channel can be treated as a single-ended, ground referenced input for one conversion; then it can be
reconfigured as part of a differential channel for another conversion.
The analog input voltages for each channel can range from 50mV below ground to 50mV above V
CC
(typically
5V) without degrading conversion accuracy.
THE DIGITAL INTERFACE
A most important characteristic of these converters is their serial data link with the controlling processor. Using a
serial communication format offers two very significant system improvements. It allows many functions to be
included in a small package and it can eliminate the transmission of low level analog signals by locating the
converter right at the analog sensor; transmitting highly noise immune digital data back to the host processor.
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