AD9280-EB datasheet(12/25 Pages) AD

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AD9280

–11–

The actual reference voltages used by the internal circuitry of

the AD9280 appear on REFTF and REFBF. For proper opera-

tion, it is necessary to add a capacitor network to decouple these

pins. The REFTF and REFBF should be decoupled for all

internal and external configurations as shown in Figure 17.

AD9280

REFTF

REFBF

0.1 F

10 F

0.1 F

Figure 17. Reference Decoupling Network

Note: REFTF = reference top, force

REFBF = reference bottom, force

REFTS = reference top, sense

REFBS = reference bottom, sense

INTERNAL REFERENCE OPERATION

Figures 18, 19 and 20 show sample connections of the AD9280

internal reference in its most common configurations. (Figures

18 and 19 illustrate top/bottom mode while Figure 20 illustrates

center span mode). Figure 29 shows how to connect the AD9280

for 1 V p-p differential operation. Shorting the VREF pin

directly to the REFSENSE pin places the internal reference

amplifier, A1, in unity-gain mode and the resultant reference

output is 1 V. In Figure 18 REFBS is grounded to give an input

range from 0 V to 1 V. These modes can be chosen when the

supply is either +3 V or +5 V. The VREF pin must be bypassed to

AVSS (analog ground) with a 1.0

µF tantalum capacitor in

parallel with a low inductance, low ESR, 0.1

µF ceramic capacitor.

MODE

AVDD

10k

A/D

CORE

4.2k

TOTAL

REFTS

REFBS

10 F

0.1 F

REFTF

REFBF

0.1 F

AIN

0.1 F

AD9280

10k

REF

SENSE

VREF

SHA

0.1 F

1.0 F

Figure 18. Internal Reference—1 V p-p Input Span

(Top/Bottom Mode)

Figure 19 shows the single-ended configuration for 2 V p-p

operation. REFSENSE is connected to GND, resulting in a 2 V

reference output.

MODE

AVDD

10k

A/D

CORE

4.2k

TOTAL

REFTS

REFBS

10 F

0.1 F

REFTF

REFBF

0.1 F

AIN

0.1 F

AD9280

10k

REF

SENSE

VREF

SHA

0.1 F

1.0 F

Figure 19. Internal Reference, 2 V p-p Input Span

(Top/Bottom Mode)

Figure 20 shows the single-ended configuration that gives the

good high frequency dynamic performance (SINAD, SFDR).

To optimize dynamic performance, center the common-mode

voltage of the analog input at approximately 1.5 V. Connect the

shorted REFTS and REFBS inputs to a low impedance 1.5 V

source. In this configuration, the MODE pin is driven to a volt-

age at midsupply (AVDD/2).

Maximum reference drive is 1 mA. An external buffer is re-

quired for heavier loads.

AVDD/2

+1.5V

MODE

10k

A/D

CORE

4.2k

TOTAL

REFTS

REFBS

10 F

0.1 F

REFTF

REFBF

0.1 F

AIN

0.1 F

AD9280

10k

REF

SENSE

VREF

SHA

0.1 F

1.0 F

Figure 20. Internal Reference 1 V p-p Input Span

(Center Span Mode)

REV. E

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