AD9480 datasheet(18/29 Pages) AD

18 / 29 page

AD9480

Rev. A | Page 17 of 28

The AD9480 can be easily configured for different full-scale

ranges. See the Voltage Reference section for more information.

Optimal performance is achieved with a 1 V p-p analog input.

SENSE = GND

VIN+

2.0V

500mV

2.0V

VIN–

DIGITALOUT = ALL 1s

DIGITALOUT = ALL 0s

Figure 33. Analog Input Full Scale

VOLTAGE REFERENCE

A stable and accurate 1.0 V reference is built into the AD9480.

Users can choose this internal reference or provide an external

reference for greater accuracy and flexibility. Figure 35 shows

the typical reference variation with temperature. Table 9

summarizes the available reference configurations.

SELECT

LOGIC

ADC

CORE

0.5V

Ω

VREF

0.1

µF

SENSE

VIN+

VIN–

Ω

Figure 34. Internal Reference Equivalent Circuit

Fixed Reference

The internal reference can be configured for a differential span

of 1 V p-p (see Figure 37). It is recommended to place a 0.1 µF

capacitor as close as possible to the VREF pin; a 10 µF capacitor

is also required (see the PCB layout for guidance). If the

internal reference of the AD9480 is used to drive multiple

converters to improve gain matching, the loading of the

reference by the other converters must be considered. Figure 37

depicts how the internal reference voltage is affected by loading.

TEMPERATURE (

°C)

1.0085

1.0080

1.0075

1.0060

1.0055

1.0065

1.0070

1.0050

1.0045

1.0040

1.0035

–40

–20

Figure 35. Typical Reference Variation with Temperature

0.1

µF

VREF

SENSE

Figure 36. Internal Fixed Reference (1 V p-p)

IREF (mA)

–0.1

–0.2

–0.4

–0.3

–0.5

0.5

1.5

1.0

2.0

2.5

3.0

Figure 37. Internal VREF vs. Load Current

Table 9. Reference Configurations

SENSE Voltage

Resulting VREF

Reference

Differential Span

AVDD

N/A (External Reference Input)

External

1 × External Reference Voltage

0.5 V (Self-Biased)

0.5 × (1 + R1/R2) V

Programmable

1 × VREF (0.75 V p-p to 1.5 V p-p)

AGND to 0.2 V

1.0 V

Internal Fixed

1 V p-p

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