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Data Sheet

AD9680

Rev. C | Page 41 of 97

Absolute Maximum Input Swing

The absolute maximum input swing allowed at the inputs of the

AD9680 is 4.3 V p-p differential. Signals operating near or at

this level can cause permanent damage to the ADC.

VOLTAGE REFERENCE

A stable and accurate 1.0 V voltage reference is built into the

AD9680. This internal 1.0 V reference is used to set the full-

scale input range of the ADC. The full-scale input range can

be adjusted via the ADC Function Register 0x025. For more

information on adjusting the input swing, see Table 36. Figure 131

shows the block diagram of the internal 1.0 V reference controls.

ADC

CORE

FULL-SCALE

VOLTAGE

ADJUST

V_1P0 PIN

CONTROL SPI

(0x025, 0x02,

AND 0x024)

V_1P0

VIN–A/

VIN–B

VIN+A/

VIN+B

INTERNAL

V_1P0

GENERATOR

INPUT FULL-SCALE

RANGE ADJUST

SPI REGISTER

(0x025, 0x02,

AND 0x024)

Figure 131. Internal Reference Configuration and Controls

The SPI Register 0x024 enables the user to either use this internal

1.0 V reference, or to provide an external 1.0 V reference. When

using an external voltage reference, provide a 1.0 V reference.

The full-scale adjustment is made using the SPI, irrespective of

the reference voltage. For more information on adjusting the

full-scale level of the AD9680, refer to the Memory Map Register

Table section.

The use of an external reference may be necessary, in some

applications, to enhance the gain accuracy of the ADC or to

improve thermal drift characteristics. Figure 132 shows the

typical drift characteristics of the internal 1.0 V reference.

–50

TEMPERATURE (°C)

0.9998

0.9999

1.0000

1.0001

1.0002

1.0003

1.0004

1.0005

1.0006

1.0007

1.0008

1.0009

1.0010

Figure 132. Typical V_1P0 Drift

The external reference must be a stable 1.0 V reference. The

ADR130 is a good option for providing the 1.0 V reference.

Figure 133 shows how the ADR130 can be used to provide the

external 1.0 V reference to the AD9680. The grayed out areas

show unused blocks within the AD9680 while using the

ADR130 to provide the external reference.

FULL-SCALE

VOLTAGE

ADJUST

V_1P0

0.1µF

VOUT 4

SET 5

NC 6

VIN

GND

ADR130

0.1µF

INPUT

FULL-SCALE

CONTROL

INTERNAL

V_1P0

GENERATOR

Figure 133. External Reference Using ADR130

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