AD6672 datasheet(17/32 Pages) AD

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AD6672

Rev. 0 | Page 17 of 32

Differential Input Configurations

Optimum performance can be achieved when driving the AD6672

in a differential input configuration. For baseband applications, the

AD8138, ADA4937-1, and ADA4930-1 differential drivers provide

excellent performance and a flexible interface to the ADC.

The output common-mode voltage of the ADA4930-1 is easily

set with the VCM pin of the AD6672 (see Figure 25), 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

AVDD

33Ω

15Ω

5pF

15pF

ADC

VIN–

VIN+

VCM

ADA4930-1

0.1µF

Figure 25. Differential Input Configuration Using the ADA4930-1

For baseband applications where SNR is a key parameter,

differential transformer coupling is the recommended input

configuration. An example is shown in Figure 26. 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

Figure 26. 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 AD6672. For applications where

SNR is a key parameter, differential double balun coupling is

the recommended input configuration (see Figure 28). In this

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

provided to the 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 26

and Figure 28.

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 AD6672.

Figure 27 shows an example of the AD8375 driving the AD6672

through a band-pass antialiasing filter.

AD8375

AD6672

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 (0603LS).

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

CENTERED AT 140MHz.

180nH

220nH

Figure 27. Differential Input Configuration Using the AD8375

ADC

0.1µF

2V p-p

VIN+

VIN–

VCM

0.1µF

33Ω

0.1µF

Figure 28. Differential Double Balun Input Configuration

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