AD9650 datasheet(31/44 Pages) AD

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AD9650

Rev. 0 | Page 31 of 44

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 AD9650. For applications in

which SNR is a key parameter, differential double balun coupling

is the recommended input configuration (see Figure 81). 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 fre-

quency and source impedance and may need to be reduced or

removed. Table 10 displays recommended values to set the RC

network. At higher input frequencies, good performance can be

achieved by using a ferrite bead in series with a resistor and

removing the capacitors. However, these values are dependent

on the input signal and should be used only as a starting guide.

Table 10. Example RC Network

Frequency

Range

(MHz)

R1 Series

(Ω Each)

C1 Differential

(pF)

R2 Series

(Ω Each)

C2 Shunt

(pF Each)

0 to 100

100 to 200

100 to 300

101

Remove

1 In this configuration, R1 is a ferrite bead with a value of 10 Ω at 100 MHz.

An alternative to using a transformer-coupled input at fre-

quencies in the second Nyquist zone is to use the AD8352

differential driver. An example is shown in Figure 82. See the

AD8352 data sheet for more information.

AD9650

0.1µF

2V p-p

VIN+x

VIN–x

VCM

0.1µF

33Ω

Figure 81. Differential Double Balun Input Configuration

AD8352

0Ω

0.1µF

8, 13

VCC

200Ω

ANALOG INPUT

AD9650

VIN+x

VIN–x

VCM

Figure 82. Differential Input Configuration Using the AD8352

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