ADS1259
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SBAS424C –JUNE 2009–REVISED MARCH 2010
Reference Drift
For external reference applications, place a 1mF
(minimum) capacitor close to the VREFP and VREFN
pins.
The ADS1259 internal reference is designed for
minimal drift error, which is defined as the change in
reference voltage over temperature. The drift is
calculated using the box method, as described by
Equation 2.
Because the ADS1259 measures the signal inputs
(AINP and AINN) against the reference inputs
(VREFP and VREFN), reference noise and drift may
degrade overall system performance. In ratiometric
measurement applications, reference noise and drift
have a cancelling effect. In absolute measurement
applications, reference noise and drift directly effect
the conversion results.
V
REFMAX - VREFMIN
Drift =
x 106 (ppm)
V
REFNOM ´ Temp Range
Where:
VREFMAX, VREFMIN, and VREFNOM are the maximum,
minimum, and nominal reference output voltages,
respectively, over the specified temperature
Voltage Reference Inputs (VREFP, VREFN)
ESD diodes protect the reference inputs. To keep
these diodes from turning on, make sure the voltages
on the reference pins do not go below AVSS by more
than 300mV, and likewise do not exceed AVDD by
more than 300mV.
range.
(2)
The ADS1259 internal reference features a maximum
drift coefficient of 5ppm/°C over 0°C to +85°C
operating range and 12ppm/°C over –40°C to +105°C
operating range.
The absolute maximum reference input range is:
External Reference
AVSS – 300mV < (VREFP or VREFN) < AVDD + 300mV(3)
To select the ADS1259 for external reference
operation, set the EXTREF register bit = 1 (default). If
desired, the internal reference can continue to
provide a +2.5V reference output via the REFOUT
and AVSS pins. In this case, set the RBIAS register
bit = 1 to power the internal reference. If the internal
reference is activated, an external 1mF capacitor from
REFOUT to AVSS is required.
Note that the valid operating range of the reference
inputs are shown in the Electrical Characteristics
table.
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