AD9691 datasheet(21/73 Pages) AD

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AD9691

Data Sheet

Rev. 0 | Page 20 of 72

1400.3

1500.3

1600.3

1700.3

1800.3

1900.3

1990.3

ANALOG INPUT FREQUENCY (MHz)

5.5x

6.5x

7.5x

8.5x

Figure 46. Buffer Current Sweeps, SFDR vs. Analog Input Frequency vs. IBUFF;

1300 MHz < fIN < 2000 MHz

Table 10 shows the recommended buffer current and full-scale

voltage settings for the different analog input frequency ranges.

Table 10. SFDR Optimization for Input Frequencies

Input Frequency

Input Buffer Current

Control Setting

(Register 0x018)

Buffer Control 2

(Register 0x935)

<500 MHz

3.5×

0x04

500 MHz to 1 GHz

5.5× or 6.5×

0x00

>1 GHz

6.5× or higher

0x00

Absolute Maximum Input Swing

The absolute maximum input swing allowed at the inputs of the

AD9691 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

AD9691. This internal 1.0 V reference sets the full-scale input

range of the ADC. For more information on adjusting the input

swing, see Table 35. Figure 47 shows the block diagram of the

internal 1.0 V reference controls.

ADC

CORE

FULL-SCALE

VOLTAGE

ADJUST

V_1P0 PIN

CONTROL SPI

(REG 0x024)

V_1P0

VIN–A/

VIN–B

VIN+A/

VIN+B

INTERNAL

V_1P0

GENERATOR

V_1P0 ADJUST

SPI REGISTER

(REG 0x024)

Figure 47. Internal Reference Configuration and Controls

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 AD9691, see 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

improve thermal drift characteristics. Figure 48 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 48. 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 49 shows how the ADR130 can be used to provide the

external 1.0 V reference to the AD9691. The grayed out areas

show unused blocks within the AD9691 when using the

ADR130 to provide the external reference.

V_1P0

ADJUST

V_1P0

0.1µF

VOUT 4

SET 5

NC 6

VIN

GND

ADR130

0.1µF

INPUT

V_1P0

ADJUST

INTERNAL

V_1P0

GENERATOR

Figure 49. External Reference Using the ADR130

AD9691 Datasheet(PDF) 21 Page - Analog Devices

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