Datasheet

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SBAS317E − APRIL 2004 − REVISED MAY 2006
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23
ADC INPUT MULTIPLEXER
The input multiplexer provides for any combination of
differential inputs to be selected as the input channel, as
shown in Figure 8. For example, if AIN0 is selected as the
positive differential input channel, then any other channel
can be selected as the negative differential input channel.
With this method, it is possible to have up to six fully
differential input channels. It is also possible to switch the
polarity of the differential input pair to negate any offset
voltages. In addition, current sources are supplied that will
source or sink current to detect open or short circuits on the
pins.
AIN3
AIN4
AIN5
AIN0
AIN1
AIN2
AINCOM
NOTE: (1) For MSC1201/MSC1202, AIN6 and AIN7 are tied to REFIN
−
.
MSC1200 Only
BurnoutDetect(2
µ
A)
BurnoutDetect(2
µ
A)
AGND
Buffer
Temperature Sensor
I
80
•
I
AV
DD
AV
DD
AV
DD
In+
In
−
AIN6
(1)
AIN7
(1)
Figure 8. Input Multiplexer Configuration
TEMPERATURE SENSOR
On-chip diodes provide temperature sensing capability.
When the configuration register for the input mux is set to
all 1s, the diodes are connected to the inputs of the ADC.
All other channels are open. The internal device power
dissipation affects the temperature sensor reading. It is
recommended that the internal buffer be enabled for
temperature sensor measurements.
BURNOUT DETECT
When the Burnout Detect (BOD) bit is set in the ADC
control configuration register (ADCON0, SFR DCh), two
current sources are enabled. The current source on the
positive input channel sources approximately 2µA of
current. The current source on the negative input channel
sinks approximately 2µA. These current sources allow for
the detection of an open circuit (full-scale reading) or short
circuit (small differential reading) on the selected input
differential pair. The buffer should be on for sensor burnout
detection.
ADC INPUT BUFFER
The analog input impedance is always high, regardless of
PGA setting (when the buffer is enabled). With the buffer
enabled, the input voltage range is reduced and the analog
power-supply current is higher. If the limitation of input
voltage range is acceptable, then the buffer is always
preferred.
The input impedance of the MSC120x without the buffer is
7MΩ/PGA. The buffer is controlled by the state of the BUF
bit in the ADC control register (ADCON0, SFR DCh).