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AD9629

Data Sheet

Rev. B | Page 18 of 32

Differential Input Configurations

Optimum performanceis achieved while drivingthe AD9629 in a

differential input configuration. For baseband applications, the

AD8138, ADA4937-2, and ADA4938-2 differential drivers provide

excellent performance and a flexible interface to the ADC.

The output common-mode voltage of the ADA4938-2 is easily

set with the VCM pin of the AD9629 (see Figure 37), and the

driver can be configured in a Sallen-Key filter topology to

provide band limiting of the input signal.

AVDD

VIN

76.8Ω

120Ω

0.1µF

33Ω

10pF

200Ω

90Ω

ADA4938

ADC

VIN–

VIN+

VCM

Figure 37. Differential Input Configuration Using the ADA4938-2

For baseband applications below ~10 MHz where SNR is a key

parameter, differential transformer-couplingis the recommended

input configuration. An example is shown in Figure 38. To bias

the analog input, the VCM voltage can be connected to the

center tap of the secondary winding of the transformer.

2V p-p

49.9Ω

0.1µF

ADC

VCM

VIN+

VIN–

Figure 38. Differential Transformer-Coupled Configuration

The signal characteristics must be considered when selecting

a transformer. Most RF transformers saturate at frequencies

below a few megahertz (MHz). Excessive signal power can

also cause core saturation, which leads to distortion.

At input frequencies in the second Nyquist zone and above, the

noise performance of most amplifiers is not adequate to achieve

the true SNR performanceof theAD9629.For applications above

~10 MHz where SNR is a key parameter, differential double balun

coupling is the recommended input configuration (see Figure 40).

An alternative to usinga transformer-coupled inputat frequencies

in the second Nyquist zone is to use the AD8352 differential driver.

An example is shown in Figure 41. See the AD8352 data sheet

for more information.

In any configuration, thevalue ofShuntCapacitor C is dependent

on the input frequency and source impedance and may need to

be reduced or removed. Table 9 displays the suggested values to set

the RC network. However, these values are dependent on the

input signal and should be used only as a starting guide.

Table 9. Example RC Network

Frequency Range (MHz)

R Series

(Ω Each)

C Differential (pF)

0 to 70

70 to 200

125

Open

Single-Ended Input Configuration

A single-ended input can provide adequate performance in

cost-sensitive applications. In this configuration, SFDR and

distortion performancedegrade due to thelarge inputcommon-

mode swing. If the source impedances oneach input are matched,

there should be little effect on SNR performance. Figure 39

shows a typical single-ended input configuration.

1V p-p

49.

9Ω

0.1µF

10µF

0.1µF

AVDD

kΩ

ADC

AVDD

VIN+

VIN–

Figure 39. Single-Ended Input Configuration

ADC

0.1µF

2V p-p

VCM

0.1µF

25Ω

VIN+

VIN–

Figure 40. Differential Double Balun Input Configuration

AD8352

0Ω

0.1µF

8, 13

VCC

200Ω

ANALOG INPUT

ADC

VCM

VIN+

VIN–

Figure 41. Differential Input Configuration Using the AD8352

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