AD7626 datasheet(20/30 Pages) AD

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Data Sheet

AD7626

Rev. D | Page 19 of 29

DRIVING THE AD7626

Differential Analog Input Source

Figure 33 shows an ADA4899-1 driving each differential input

to the AD7626.

Single-Ended to Differential Driver

For applications using unipolar analog signals, a single-ended to

differential driver (as shown in Figure 32) allows for a differential

input into the device. This configuration, when provided with

an input signal of 0 V to 4.096 V, produces a differential ±4.096 V

with midscale at 2.048 V. The one-pole filter using R = 20 Ω and

C = 56 pF provides a corner frequency of 140 MHz. The VCM

output of the AD7626 can be buffered and then provide the

required 2.048 V common-mode voltage.

Single-Ended or Fully Differential High Frequency Driver

In applications that require higher input frequency tones, the

ADA4932-1 can drive the inputs to the AD7626. The ADA4932-1

is a differential driver, which also allows the user the option of

single-ended to differential conversion.

Figure 34 shows the typical circuit for a 50 Ω source impedance

(ac-coupled in this example). The input to the ADA4932-1 is

configured to be balanced to the source impedance (in this case

50 Ω). Further information on balancing the input impedance

to the source impedance can be found on the ADA4932-1 data

sheet. The circuit shown in Figure 34 operates with an overall gain

of ~0.5 when the termination input termination is taken

into account.

Alternatively, the ADA4932-1 can be used with a fully differential

source—it acts as an inverting differential driver.

56pF

20Ω

50Ω

IN+

IN–

AD7626

VCM

V–

V+

AD8031, AD8032

ADA4899-1

56pF

100nF

590Ω

ANALOG INPUT

(UNIPOLAR 0V TO 4.096V)

Figure 32. Single-Ended to Differential Driver Circuit Using ADA4899-1

56pF

20Ω

–VS

0V TO VREF

+VS

56pF

20Ω

–VS

VREF TO 0V

+VS

CREF

10µF2

REF1

REF

GND

IN+

IN–

AD7626

ADA4899-1

1SEE THE VOLTAGE REFERENCE OPTIONS SECTION. CONNECTION TO EXTERNAL REFERENCE SIGNALS

IS DEPENDENT ON THE EN1 AND EN0 SETTINGS.

2CREF IS USUALLY A 10µF CERAMIC CAPACITOR WITH LOW ESL AND ESR.

DECOUPLE REF AND REFIN PINS AS PER THE EN1 AND EN0 RECOMMENDATIONS

VCM

–VS

+VS

BUFFERED VCM PIN OUTPUT

GIVES THE REQUIRED 2.048V

COMMON-MODE SUPPLY FOR

ANALOG INPUTS.

VCM

2.048V

AD8031, AD8032

ADA4899-1

REFIN

REF1

CREF

10µF2

0.1µF

Figure 33. Driving the AD7626 from a Differential Analog Source Using ADA4899-1

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