Datasheet

AD9248
Rev. B | Page 21 of 48
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. When multiple ADCs track one another, a
single reference (internal or external) may be necessary to
reduce gain matching errors to an acceptable level. A high
precision external reference may also be selected to provide
lower gain and offset temperature drift. Figure 37 shows the
typical drift characteristics of the internal reference in both
1 V and 0.5 V modes. When the SENSE pin is tied to AVDD,
the internal reference is disabled, allowing the use of an
external reference. An internal reference buffer loads the
external reference with an equivalent 7 kΩ load. The internal
buffer still generates the positive and negative full-scale
references, REFT and REFB, for the ADC core. The input span
is always twice the value of the reference voltage; therefore, the
external reference must be limited to a maximum of 1 V. If the
internal reference of the AD9248 is used to drive multiple
converters to improve gain matching, the loading of the
reference by the other converters must be considered. Figure 38
depicts how the internal reference voltage is affected by loading.
VIN+
VIN–
VREF
REFT
SENSE
0.5V
AD9248
REFB

R1
R2
10μF
10μF
0.1μF
0.1μF
10μF
ADC
CORE
SELECT
LOGIC
0.1μF
04446-035
Figure 36. Programmable Reference Configuration
TEMPERATURE (°C)
0.2
VREF ERROR (%)
1.2
1.0
0.8
0.6
0.4
0
–40
–30
–20 –10
0
10
20
30
40
50 60
70
80
VREF = 1V
VREF = 0.5V
04446-036
Figure 37. Typical VREF Drift
LOAD (mA)
–0.20
ERROR (%)
0.05
0
–0.05
–0.10
–0.15
–0.25
0
0.5
1.0
1.5
2.0
2.5
3.0
0.5V ERROR
1V ERROR
04446-037
Figure 38. VREF Accuracy vs. Load