AD9644_17 datasheet(22/45 Pages) AD

22 / 45 page

Data Sheet

AD9644

Rev. C | Page 21 of 44

Differential Input Configurations

Optimum performance is achieved while driving the AD9644 in a

differential input configuration. For baseband applications, the

AD8138, ADA4937-2, and ADA4938-2 differential drivers provide

excellent performance and a flexible interface to the ADC.

The output common-mode voltage of the ADA4938-2 is easily

set with the VCM pin of the AD9644 (see Figure 49), 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

ADA4938-2

Figure 49. Differential Input Configuration Using the ADA4938-2

For baseband applications in which SNR is a key parameter,

differential transformer coupling is the recommended input

configuration. An example is shown in Figure 50. 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

Figure 50. Differential Transformer-Coupled Configuration

The signal characteristics must be considered when selecting

a transformer. Most RF transformers saturate at frequencies

below a few megahertz (MHz). 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 AD9644. For applications in

which SNR is a key parameter, differential double balun coupling

is the recommended input configuration (see Figure 51). In this

configuration, the input is ac-coupled and the VCM 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 50 and Figure 51.

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

Open

An alternative to using a transformer-coupled input at frequencies

in the second Nyquist zone is to use the AD8376 variable gain

amplifier. An example drive circuit including a band-pass filter

is shown in Figure 52. See the AD8376 data sheet for more

information.

ADC

0.1µF

2V p-p

VIN+

VIN–

VCM

0.1µF

33Ω

Figure 51. Differential Double Balun Input Configuration

AD9644 Datasheet(PDF) 22 Page - Analog Devices

AD9644 Datasheet(HTML) 22 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 37 38 39 40 41 42 43 44 45

Datasheet Download

Link URL