Datasheet

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SBAS306F − NOVEMBER 2004 − REVISED OCTOBER 2007
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17
VOLTAGE REFERENCE INPUTS (VREFP,
VREFN)
The voltage reference for the ADS1271 ADC is the
differential voltage between VREFP and VREFN:
V
REF
= (VREFP−VREFN). The reference inputs use a
structure similar to that of the analog inputs with the
equivalent circuitry on the reference inputs shown in
Figure 47. As with the analog inputs, the load presented by
the switched capacitor can be modeled with an effective
impedance, as shown in Figure 48.
ESD
Protection
AVDDAVDD
VREFN
VREFP
Figure 47. Equivalent Reference Input Circuitry
VREFP VREFN
Zeff = 4.2k
Ω×
(6.75MHz/f
MOD
)
Figure 48. Effective Reference Impedance
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 AGND by more than 0.4V,
and likewise do not exceed AVDD by 0.4V. If these
conditions are possible, external Schottky clamp diodes or
series resistors may be required to limit the input current
to safe values (see Absolute Maximum Ratings).
Note that the valid operating range of the reference inputs
is limited to the following:
For the ADS1271:
−0.1V ≤ VREFN ≤ VREFP − 2V
VREFN + 2V ≤ VREFP ≤ AVDD − 0.5V
For the ADS1271B:
−0.1V ≤ VREFN ≤ VREFP − 0.5V
VREFN + 0.5V ≤ VREFP ≤ AVDD + 0.1V
A high-quality reference voltage with the appropriate drive
strength is essential for achieving the best performance from
the ADS1271. Noise and drift on the reference degrade
overall system performance. See the Application Information
section for example reference circuits.