AD9238 datasheet(21/49 Pages) AD

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AD9238

Rev. C | Page 20 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 AD9238 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

μF

0.1

μF

0.1

μF

REFT

ADC

CORE

SELECT

LOGIC

SENSE

0.5V

AD9238

REFB

0.1

μF

Figure 36. Programmable Reference Configuration

TEMPERATURE (

°C)

0.2

1.2

1.0

0.8

0.6

0.4

–40 –30 –20 –10

VREF = 1V

VREF = 0.5V

Figure 37. Typical VREF Drift

LOAD (mA)

–0.20

0.05

–0.05

–0.10

–0.15

–0.25

0.5

1.0

1.5

2.0

2.5

3.0

0.5V ERROR

1V ERROR

Figure 38. VREF Accuracy vs. Load

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