ADS1259
SBAS424C –JUNE 2009–REVISED MARCH 2010
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REFERENCE
the ADC reference input pins, VREFP and VREFN.
(Note that these device pins are not intended to drive
external circuits.) An external 1mF capacitor,
connected from VREFP to VREFN, is recommended
for noise reduction. The capacitor can be increased
for increased noise filtering, but the settling time of
the reference may also increase. The settling time
should be considered upon activating the internal
reference.
The ADS1259 includes an onboard voltage reference
with a low temperature coefficient. The reference
voltage is 2.5V with the capability of sinking and
sourcing 10mA via the REFOUT pin. The ADS1259
can also operate from an external reference. The
external reference is the default selection. Refer to
Figure 37 for a reference block diagram.
Internal Reference
See Figure 29 for typical reference settling CREFIN =
1µF. The capacitor dielectric absorption results in
increased settling time for RC filter circuits.
The reference output is provided between pins
REFOUT and AVSS. Because the reference output
return shares the same pin as AVSS, route the
reference return trace and the AVSS trace
independently as Kelvin-connected printed circuit
board (PCB) traces. For stability reasons, connect a
1mF capacitor between REFOUT and AVSS.
To activate the internal reference, set the register bit
RBIAS = 1. This enables the reference bias. Once
biased, the internal reference can then be selected as
the ADC reference by the register bit EXTREF.
EXTREF = 0 closes the internal switches.
An internal switch connects the internal reference to
AVDD
Reference Bias
RBIAS Register Bit
(1 = Bias On)
REFOUT
(+)
2.5V
Reference
+
1mF
Reference Output
(CREFOUT
)
(-)
AVSS
Reference Select
EXTREF Register Bit
(1 = Switch Open for External Reference)
2kW
VREFP
(+)
Reference Input
(-)
(+)
+
1mF
(CREFIN
ADC Reference Input
)
(-)
VREFN
Figure 37. Reference Block Diagram
Table 2. Reference Selection for Figure 37
ADS1259 REFERENCE
Internal
RBIAS REGISTER BIT
EXTREF REGISTER BIT
1
0
1
(1)
External
See
(1) If the reference output is not required, set RBIAS = 0. If the reference output is enabled (RBIAS = 1), an external 1µF capacitor must be
used between REFOUT and AVSS.
16
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