Datasheet
AD9287 Data Sheet
Rev. E | Page 28 of 52
Internal Reference Operation
A comparator within the AD9287 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 53), setting VREF to 1 V.
The REFT and REFB pins establish the input span of the ADC
core from the reference configuration. The analog input full-
scale range of the ADC equals twice the voltage of the reference
pin for either an internal or an external reference configuration.
If the reference of the AD9287 is used to drive multiple
converters to improve gain matching, the loading of the refer-
ence by the other converters must be considered. Figure 55
depicts how the internal reference voltage is affected by loading.
1µF 0.1µF
VREF
SENSE
0.5V
REFT
0.1µF
0.1µF 2.2µF
0.1µF
REFB
SELECT
LOGIC
ADC
CORE
+
VIN – x
VIN + x
05966-010
Figure 53. Internal Reference Configuration
1µF
1
0.1µF
1
VREF
SENSE
AVDD
0.5V
REFT
0.1µF
0.1µF 2.2µF
0.1µF
REFB
SELECT
LOGIC
ADC
CORE
+
VIN – x
VIN + x
05966-089
EXTERNAL
REFERENCE
1
OPTIONAL.
Figure 54. External Reference Operation
External Reference Operation
The use of an external reference may be necessary to enhance
the gain accuracy of the ADC or to improve thermal drift
characteristics. Figure 56 shows the typical drift characteristics
of the internal reference in 1 V mode.
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 1.0 V.
0 1.00.5 2.01.5 3.02.5 3.5
V
REF
ERROR (%)
CURRENT LOAD (mA)
05966-083
–30
–5
–10
–15
–20
–25
5
0
Figure 55. V
REF
Error vs. Load
0.02
–0.18
–40 806040200–20
TEMPERATURE (°C)
V
REF
ERROR (%)
0
–0.02
–0.04
–0.06
–0.08
–0.10
–0.12
–0.14
–0.16
05966-084
Figure 56. V
REF
Error vs. Temperature