AD9613 datasheet(24/36 Pages) AD

24 / 36 page

AD9613

Data Sheet

Rev. C | Page 24 of 36

Differential Input Configurations

Optimum performance is achieved while driving the AD9613

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 AD9613 (see Figure 47), 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

ADA4930-2

Figure 47. Differential Input Configuration Using the ADA4938-2

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 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

33Ω

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 AD9613. For applications where

SNR is a key parameter, differential double balun coupling is

the recommended input configuration (see Figure 49). 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

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 10

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 10 are for

each R1, R2, C2, and R3 component shown in Figure 48 and

Figure 49.

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 AD9613.

Figure 50 shows an example of the AD8376 driving the AD9613

through a band-pass antialiasing filter.

Table 10. Example RC Network

Frequency Range (MHz)

R1 Series (Ω)

C1 Differential (pF)

R2 Series (Ω)

C2 Shunt (pF)

R3 Shunt (Ω)

0 to 100

8.2

49.9

100 to 300

3.9

8.2

49.9

ADC

0.1µF

2V p-p

VIN+

VIN–

VCM

0.1µF

33Ω

0.1µF

33Ω

Figure 49. Differential Double Balun Input Configuration

AD9613 Datasheet(PDF) 24 Page - Analog Devices

AD9613 Datasheet(HTML) 24 Page - Analog Devices

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36

Datasheet Download

Link URL