Voltage Reference
5
Voltage Reference
The ADS1281EVM has many options for supplying a reference voltage to the ADS1281. Jumper J1 can
select the voltage from the REF5050 (U1) or use ground. The REF5050 supplies 5.0V to the reference for
unipolar mode. If ground is selected, then the voltage reference becomes 2.5V in bipolar mode. This
configuration comes from the voltage difference across the VREFP and VREFN pins on the ADS1281.
The voltage is filtered and buffered by U3 before it is used as a reference for the ADS1281.
The ADS1281 can also be referenced by an external reference voltage using pins J6.18 (REF
–
) and J6.20
(REF+). Switch S1 chooses either the onboard reference or the external reference voltage that is
connected to the pins of J6.
shows switch S1 as it appears on the EVM.
Figure 2. Switch S1
6
Power-Down, Mode and Format Control
The power-down functionality of the ADS1281 is controlled by GPIO4 (J5.14) on the EVM. The
ADS1281EVM-PDK uses the I
2
C port expander on the EVM to allow the software to control this pin.
7
Clock Source
The ADS1281 clock can come from one of two sources: the onboard 4.096MHz crystal oscillator, or a
clock supplied by a processor on the TOUT pin (J5.17).
7.1
Usage in PDK
If using the ADS1281EVM as part of the ADS1281EVM-PDK, the J3 jumper should be set to the DVDD
position. Using the onboard 4.096MHz crystal oscillator allows the the ADS1281EVM-PDK software to
recognize the EVM using the software provided.
7.2
Usage as a Stand-Alone EVM
If using the EVM within your own system and not with the PDK hardware and software, jumper J3 can be
used to select the clock. The jumper can be used to always select the 4.096MHz crystal (DVDD position)
or allow the onboard clock to be controlled by GPIO5 (J5.19), as shown in
. The P05 (U7.6) pin
allows the I
2
C interface to program a clock to GPIO5.
Figure 3. Jumper J3
6
ADS1281EVM and ADS1281EVM-PDK User
'
s Guide
SBAU143C
–
March 2009
–
Revised May 2011
Copyright
©
2009
–
2011, Texas Instruments Incorporated