AD9643 datasheet(24/36 Pages) AD

24 / 36 page

AD9643

Rev. A | Page 24 of 36

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 AD9643 (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

0.1µF

AVDD

33Ω

15Ω

5pF

15pF

ADC

VIN–

VIN+

VCM

ADA4930-2

Figure 47. 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 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

0.1µF

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 AD9643. 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. 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 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 50.

Table 10. 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 or AD8376 digital variable gain amplifier

(DVGAs) provides good performance for driving the AD9643.

Figure 49 shows an example of the AD8376 driving the AD9643

through a band-pass antialiasing filter.

AD8376

AD9643

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 AD8376

ADC

0.1µF

2V p-p

VIN+

VIN–

VCM

0.1µF

33Ω

0.1µF

33Ω

Figure 50. Differential Double Balun Input Configuration

AD9643 датащи(PDF) 24 Page - Analog Devices

AD9643 датащи(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

датащи скачать

ссылки URL