![background image](http://html1.mh-extra.com/html/texas-instruments/ads1244evm/ads1244evm_user-manual_1094281022.webp)
Switches
2-9
Circuit Description
In the EXT position, J1 pins 1 and 2 are connected to the ADS1244/45 in-
puts.
In the REF position, the reference is connected across the ADS1244/45
inputs. This normally gives a full−scale reading.
In the ZERO position, both of the ADS1244/45 input pins are connected to
the reference. This gives a zero−scale reading.
The REF and ZERO positions are useful for certain noise tests.
2.3.3
S3: Reference Input Select
This switch controls which signals are connected to the ADS1244/45 differ-
ential reference input. The switch positions are described in Table 2−7.
Table 2−7. Reference Input Select Switch
Board
Marking
Switch
Position
Reference Source
ADS1244/45
REF+
ADS1244/45
REF−
EXT
Left
External (J1, pins 18 and 20)
J1 pin 20
J1 pin 18
OBH
Middle
On−board, high common−
mode
+AVDD
+2.5V
OBL
Right
Zero (shorted to reference)
+2.5V
AGND
In the EXT position, J1 pins 18 and 20 are connected to the ADS1244/45
reference input.
In the OBH position, the analog power supply is connected to the positive
reference input, and the negative reference input is connected to the output
of the onboard reference.
In the OBL position, the onboard reference output is connected to the
ADS1244/45 positive reference input pin, and the negative reference input
pin is grounded.
Both OBH and OBL provide a +2.5V reference to the ADS1244/45. The
OBL position corresponds to the usual way to connect a reference to the
ADS1244/45, and should be used for most measurements. The OBH posi-
tion is useful for testing the reference input common−mode sensitivity,
which can be important for ratiometric connections.