AD6655 datasheet(31/85 Pages) AD

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AD6655

Data Sheet

Rev. B | Page 30 of 84

An on-board common-mode voltage reference is included in

the design and is available from the CML pin. Optimum perform-

ance is achieved when the common-mode voltage of the analog

input is set by the CML pin voltage (typically 0.55 × AVDD).

Differential Input Configurations

Optimum performance is achieved while driving the AD6655

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 AD8138 is

easily set with the CML pin of the AD6655 (see Figure 47), and

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

provide band limiting of the input signal.

AVDD

1V p-p

49.9Ω

523Ω

0.1µF

499Ω

AD8138

AD6655

VIN+

VIN–

CML

Figure 47. 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 48. To bias the

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

tap of the secondary winding of the transformer.

2V p-p

49.9Ω

0.1µF

AD6655

VIN+

VIN–

CML

Figure 48. 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 performance of the AD6655. For applications where

SNR is a key parameter, differential double balun coupling is

the recommended input configuration (see Figure 49).

An alternative to using a transformer-coupled input at frequencies

in the second Nyquist zone is to use the AD8352 differential

driver is shown in Figure 50. See the AD8352 data sheet for

more information. In addition, if the application requires an

amplifier with variable gain, the AD8375 or AD8376 digital

variable gain amplifiers (DVGAs) provide good performance

driving the AD6655.

In any configuration, the value of the shunt capacitor, C, is

dependent on the input frequency and source impedance and

may need to be reduced or removed. Table 14 displays recom-

mended 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 14. Example RC Network

Frequency Range

(MHz)

R Series

(Ω, Each)

C Differential

(pF)

0 to 70

70 to 200

200 to 300

>300

Open

AD6655

0.1µF

2V p-p

VIN+

VIN–

CML

0.1µF

25Ω

Figure 49. Differential Double Balun Input Configuration

AD6655

AD8352

0Ω

0.1µF

VIN+

VIN–

CML

0.1µF

8, 13

VCC

200Ω

ANALOG INPUT

Figure 50. Differential Input Configuration Using the AD8352

AD6655 Datasheet(PDF) 31 Page - Analog Devices

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