Datasheet

AD9271
Rev. B | Page 34 of 60
External Reference Operation
Internal Reference Operation
The use of an external reference may be necessary to enhance
the gain accuracy of the ADC or to improve thermal drift charac-
teristics. Figure 69 shows the typical drift characteristics of the
internal reference in 1 V mode.
A comparator within the AD9271 detects the potential at the
SENSE pin and configures the reference. If SENSE is grounded,
the reference amplifier switch is connected to the internal
resistor divider (see Figure 66), setting VREF to 1 V.
When the SENSE pin is tied to AVDD, the internal reference is
disabled, allowing the use of an external reference. The external
reference is loaded with an equivalent 6 kΩ load. An internal
reference buffer generates the positive and negative full-scale
references, REFT and REFB, for the ADC core. Therefore, the
external reference must be limited to a nominal voltage of 1.0 V.
The REFT and REFB pins establish their input span of the ADC
core from the reference configuration. The analog input full-
scale range of the ADC equals twice the voltage at the reference
pin for either an internal or an external reference configuration.
1µF 0.1µF
VREF
SENSE
0.5V
REFT
0.1µF
0.1µF 4.7µF
0.1µF
REFB
SELECT
LOGIC
ADC
CORE
+
VIN–
VIN+
406304-06
6304-065
.5
5
0
–5
–10
–15
–20
–25
033.02.52.01.51.00.5
06304-017
VREF ERROR (%)
CURRENT LOAD (mA)
Figure 66. Internal Reference Configuration
Figure 68. VREF Accuracy vs. Load, AD9271-50
0.02
0
–0.02
–0.04
–0.06
–0.08
–0.10
–0.12
–0.14
–0.16
–0.18
–0.20
–40 806040200–20
06304-015
VREF ERROR (%)
TEMPERATURE (°C)
VIN–
VIN+
1µF*
0.1µF*
VREF
SENSE
AVDD
0.5V
REFT
0.1µF
0.1µF 4.7µF
0.1µF
REFB
SELECT
LOGIC
ADC
CORE
+
EXTERNAL
REFERENCE
*OPTIONAL.
Figure 69. Typical VREF Drift, AD9271-50
0
Figure 67. External Reference Operation