AD9640 datasheet(28/54 Pages) AD

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AD9640

Rev. B | Page 26 of 52

The output common-mode voltage of the AD8138 is easily set

with the CML pin of the AD9640 (see Figure 46), and the driver

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

band limiting of the input signal.

AVDD

1V p-p

49.9

Ω

523

Ω

0.1µF

499

Ω

499

Ω

499

Ω

AD8138

AD9640

VIN+

VIN–

CML

Figure 46. Differential Input Configuration Using the AD8138

For baseband applications where SNR is a key parameter,

differential transformer coupling is the recommended input

configuration. An example is shown in Figure 47. To bias the

analog input, the CML voltage can be connected to the center

tap of the transformer’s secondary winding.

2V p-p

49.9

Ω

0.1µF

AD9640

VIN+

VIN–

CML

Figure 47. Differential Transformer-Coupled Configuration

The signal characteristics must be considered when selecting

a transformer. Most RF transformers saturate at frequencies

below a few MHz, and 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 performance of the AD9640. For applications where

SNR is a key parameter, differential double balun coupling is the

recommended input configuration (see Figure 49 for an example).

An alternative to using a transformer coupled input at frequencies

in the second Nyquist zone is to use the AD8352 differential

driver. An example is shown in Figure 50. See the AD8352 data

sheet for more information.

In any configuration, the value of Shunt Capacitor C is dependent

on the input frequency and source impedance and may need to

be reduced or removed. Table 13 displays recommended 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 13. Example RC Network

Frequency Range (MHz)

R Series

(Ω Each)

C Differential (pF)

0 to 70

70 to 200

200 to 300

>300

Open

Single-Ended Input Configuration

A single-ended input can provide adequate performance in cost

sensitive applications. In this configuration, SFDR and distortion

performance degrade due to the large input common-mode swing.

If the source impedances on each input are matched, there should

be little effect on SNR performance. Figure 48 details a typical

single-ended input configuration.

1V p-p

49.9

Ω

0.1µF

10µF

0.1µF

AVDD

Ω

AD9640

AVDD

VIN+

VIN–

Figure 48. Single-Ended Input Configuration

AD9640

0.1µF

2V p-p

VIN+

VIN–

CML

0.1µF

Ω

Figure 49. Differential Double Balun Input Configuration

AD9640

AD8352

Ω

0.1µF

VIN+

VIN–

CML

0.1µF

8, 13

VCC

200

Ω

200

Ω

ANALOG INPUT

Figure 50. Differential Input Configuration Using the AD8352

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