AD9259 datasheet(29/53 Pages) AD

29 / 53 page

AD9259

Data Sheet

Rev. E | Page 28 of 52

Internal Reference Operation

A comparator within the AD9259 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 AD9259 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

2.2µF

0.1µF

REFB

SELECT

LOGIC

ADC

CORE

VIN – x

VIN + x

Figure 53. Internal Reference Configuration

1µF1

0.1µF1

VREF

SENSE

AVDD

0.5V

REFT

0.1µF

2.2µF

0.1µF

REFB

SELECT

LOGIC

ADC

CORE

VIN – x

VIN + x

EXTERNAL

REFERENCE

1OPTIONAL.

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.

1.0

0.5

2.0

1.5

3.0

2.5

3.5

CURRENT LOAD (mA)

–30

–5

–10

–15

–20

–25

Figure 55. VREF Accuracy vs. Load

0.02

–0.18

–40

–20

TEMPERATURE (°C)

–0.02

–0.04

–0.06

–0.08

–0.10

–0.12

–0.14

–0.16

Figure 56. Typical VREF Drift

AD9259 Datasheet(PDF) 29 Page - Analog Devices

AD9259 Datasheet(HTML) 29 Page - Analog Devices

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53

Datasheet Download

Link URL