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AD9642

Data Sheet

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, connect the VCM voltage to the center tap of the

secondary winding of the transformer.

2V p-p

49.9Ω

0.1µF

ADC

VIN+

VIN–

VCM

0.1µF

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. 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 AD9642. For applications where

SNR is a key parameter, differential double balun coupling is

the recommended input configuration (see Figure 50). In this

configuration, the input is ac-coupled and the VCM voltage is

provided to each input through a 33 Ω resistor. This resistor

compensates 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

frequency 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 48 and Figure 50.

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 300

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 digital variable gain amplifier

(DVGA) provides good performance for driving the AD9642.

Figure 49 shows an example of the AD8375 driving the AD9642

through a band-pass antialiasing filter.

AD8375

AD9642

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 (COIL CRAFT 0603LS).

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

CENTERED AT 140MHz.

180nH

220nH

Figure 49. Differential Input Configuration Using the AD8375

ADC

0.1µF

2V p-p

VIN+

VIN–

VCM

0.1µF

33Ω

0.1µF

Figure 50. Differential Double Balun Input Configuration

Rev. B | Page 18 of 28

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