AD9280-EB datasheet(13/25 Pages) AD

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AD9280

–12–

EXTERNAL REFERENCE OPERATION

Using an external reference may provide more flexibility and

improve drift and accuracy. Figures 21 through 23 show ex-

amples of how to use an external reference with the AD9280.

To use an external reference, the user must disable the internal

reference amplifier by connecting the REFSENSE pin to VDD.

The user then has the option of driving the VREF pin, or driv-

ing the REFTS and REFBS pins.

The AD9280 contains an internal reference buffer (A2), that

simplifies the drive requirements of an external reference. The

external reference must simply be able to drive a 10 k

Ω load.

Figure 21 shows an example of the user driving the top and bottom

references. REFTS is connected to a low impedance 2 V source

and REFBS is connected to a low impedance 1 V source. REFTS

and REFBS may be driven to any voltage within the supply as long

as the difference between them is between 1 V and 2 V.

AVDD

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

SHA

Figure 21. External Reference Mode—1 V p-p Input Span

Figure 22 shows an example of an external reference generating

2.5 V at the shorted REFTS and REFBS inputs. In this in-

stance, a REF43 2.5 V reference drives REFTS and REFBS. A

resistive divider generates a 1 V VREF signal that is buffered by

A3. A3 must be able to drive a 10 k

Ω, capacitive load. Choose

this op amp based on noise and accuracy requirements.

3.0V

2.0V

2.5V

AVDD

AIN

REFTS

REFTF

REFBF

REFBS

VREF

REFSENSE

MODE

AD9280

0.1 F

1.5k

10 F

0.1 F

+5V

0.1 F

10 F

AVDD

1.0 F

0.1 F

AVDD

0.1 F

AVDD/2

REF43

Figure 22. External Reference Mode—1 V p-p Input

Span 2.5 VCM

Figure 23a shows an example of the external references driving

the REFTF and REFBF pins that is compatible with the

AD876. REFTS is shorted to REFTF and driven by an external

4 V low impedance source. REFBS is shorted to REFBF and

driven by a 2 V source. The MODE pin is connected to GND

in this configuration.

0.1 F

AVDD

10 F

0.1 F

VIN

REFTS

REFTF

REFBF

REFBS

VREF

REFSENSE

MODE

AD9280

Figure 23a. External Reference—2 V p-p Input Span

+5V

0.1 F

REFTS

REFTF

10 F

0.1 F

REFBS

REFBF

0.1 F

AD9280

REFT

REFB

Figure 23b. Kelvin Connected Reference Using the AD9280

STANDBY OPERATION

The ADC may be placed into a powered down (sleep) mode by

driving the STBY (standby) pin to logic high potential and

holding the clock at logic low. In this mode the typical power

drain is approximately 4 mW.

The ADC will “wake up” in 400 ns (typ) after the standby pulse

goes low.

CLAMP OPERATION

The AD9280ARS features an optional clamp circuit for dc

restoration of video or ac coupled signals. Figure 24 shows the

internal clamp circuitry and the external control signals needed

for clamp operation. To enable the clamp, apply a logic high to

the CLAMP pin. This will close the switch SW1. The clamp

amplifier will then servo the voltage at the AIN pin to be equal

to the clamp voltage applied at the CLAMPIN pin. After the

desired clamp level is attained, SW1 is opened by taking

CLAMP back to a logic low. Ignoring the droop caused by the

input bias current, the input capacitor CIN will hold the dc

voltage at AIN constant until the next clamp interval. The input

resistor RIN has a minimum recommended value of 10

Ω, to

maintain the closed-loop stability of the clamp amplifier.

REV. E

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