AD6649EBZ datasheet(18/40 Pages) AD

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AD6649

Rev. 0 | Page 18 of 40

Differential Input Configurations

Optimum performance is achieved while driving the AD6649

in a differential input configuration. For baseband applications,

the AD8138, ADA4937-2, ADA4938-2, and ADA4930-2

differential drivers provide excellent performance and a flexible

interface to the ADC.

The output common-mode voltage of the ADA4930-2 is easily

set with the VCM pin of the AD6649 (see Figure 27), and the

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

provide band-limiting of the input signal.

VIN

76.8Ω

120Ω

0.1µF

200Ω

90Ω

33Ω

15Ω

5pF

15pF

0.1µF

15pF

33Ω

ADC

VIN–

VIN+

VCM

ADA4930-2

Figure 27. Differential Input Configuration Using the ADA4930-2

For baseband applications where SNR is a key parameter,

differential transformer coupling is the recommended input

configuration. An example is shown in Figure 28. 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

VIN+

VIN–

VCM

0.1µF

33Ω

Figure 28. Differential Transformer-Coupled Configuration

The signal characteristics must be considered when selecting

a transformer. Most RF transformers saturate at frequencies

below a few megahertz. 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 AD6649. For applications where

SNR is a key parameter, differential double balun coupling is

the recommended input configuration (see Figure 30). In this

configuration, the input is ac-coupled and the CML is provided

to each input through a 33 Ω resistor. These resistors compensate

for losses in the input baluns to provide a 50 Ω impedance to

the driver.

In the double balun and transformer configurations, the value of

the input capacitors and resistors is dependent on the input fre-

quency and source impedance. Based on these parameters the

value of the input resistors and capacitors may need to be

adjusted or some components may need to be removed. Table 9

displays recommended values to set the RC network for different

input frequency ranges. However, these values are dependent on

the input signal and bandwidth and should be used only as a

starting guide. Note that the values given in Table 9 are for each

R1, R2, C2, and R3 component shown in Figure 28 and Figure 30.

Table 9. Example RC Network

Frequency

Range

(MHz)

Series

(Ω)

Differential

(pF)

Series

(Ω)

Shunt

(pF)

Shunt

(Ω)

0 to 100

8.2

49.9

100 to 250

3.9

8.2

49.9

An alternative to using a transformer-coupled input at frequencies

in the second Nyquist zone is to use an amplifier with variable

gain. The AD8375 or AD8376 digital variable gain amplifier

(DVGAs) provides good performance for driving the AD6649.

Figure 29 shows an example of the AD8376 driving the AD6649

through a band-pass antialiasing filter.

AD8376

AD6649

1µH

1nF

VPOS

VCM

15pF

68nH

2.5kΩ║2pF

301Ω

165Ω

5.1pF

3.9pF

180nH

1000pF

NOTES

1. ALL INDUCTORS ARE COILCRAFT® 0603CS COMPONENTS

WITH THE EXCEPTION OF THE 1µH CHOKE INDUCTORS (COILCRAFT 0603LS).

2. FILTER VALUES SHOWN ARE FOR A 20MHz BANDWIDTH FILTER

CENTERED AT 140MHz.

180nH

220nH

Figure 29. Differential Input Configuration Using the AD8376

ADC

0.1µF

2V p-p

VIN+

VIN–

VCM

0.1µF

33Ω

0.1µF

33Ω

Figure 30. Differential Double Balun Input Configuration

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