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SBAS288D − JUNE 2003 − REVISED AUGUST 2004
However, for optimum analog performance, the following
recommendations are made:
INPUT MULTIPLEXER
Figure 6 shows a simplified diagram of the input
multiplexer. This flexible block allows any analog input pin
to be connected to either of the converter differential
inputs. That is, any pin can be selected as the positive
input (AINP); likewise, any pin can be selected as the
negative input (AINN). The pin selection is controlled by
the multiplexer register.
1. For differential measurements use AIN0 through
AIN7, preferably adjacent inputs. For example, use
AIN0 and AIN1. Do not use AINCOM.
2. For single-ended measurements use AINCOM as
common input and AIN0 through AIN7 as
single-ended inputs.
3. Leave any unused analog inputs floating. This
minimizes the input leakage current.
ESD diodes protect the analog inputs. To keep these
diodes from turning on, make sure the voltages on the
input pins do not go below AGND by more than 100mV,
and likewise do not exceed AVDD by more than 100mV:
−100mV < (AIN0 − 7 and AINCOM) < AVDD + 100mV.
The ADS1256 offers nine analog inputs, which can be
configured as four independent differential inputs, eight
single-ended inputs, or a combination of differential and
single-ended inputs.
The ADS1255 offers three analog inputs, which can be
configured as one differential input or two single-ended
inputs. When using the ADS1255 and programming the
input, make sure to select only the available inputs when
programming the input multiplexer register.
When using ADS1255/6 for single-ended measurements,
it is important to note that common input AINCOM does not
need to be tied to ground. For example, AINCOM can be
tied to a midpoint reference such as +2.5V or even AVDD.
In general, there are no restrictions on input pin selection.
AVDD
AIN0
AVDD
AIN1
AVDD
AVDD
Sensor Detect
Current
Source
AIN2
AVDD
AIN3
AVDD
AINP
Input
Buffer
AINN
AIN4
AVDD
AIN5
AVDD
Sensor Detect
Current
AIN6
Source
AVDD
AGND
AIN7
ADS1256 Only
AINCOM
Input Multiplexer
AVDD AGND
Figure 6. Simplified Diagram of the Input Multiplexer
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