AD9640SLASHPCB datasheet(18/41 Pages) AD

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AD9640

Preliminary Technical Data

Rev. PrD | Page 18 of 41

voltage of the AD8352 is easily set with the CML pin of the

AD9640 (see Figure 4), and the driver can be configured in a

Sallen-Key filter topology to provide band limiting of the input

signal.

Figure 4. Differential Input Configuration Using the AD8138

For baseband applications where SNR is a key parameter,

differential transformer coupling is the recommended input

configuration. An example is shown in Figure 5. The CML

voltage can be connected to the center tap of the transformer’s

secondary winding to bias the analog input.

The signal characteristics must be considered when selecting a

transformer. Most RF transformers saturate at frequencies

below a few MHz, and excessive signal power can also cause

core saturation, which leads to distortion.

Figure 5. Differential Transformer-Coupled Configuration

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 AD9640. For applications

where SNR is a key parameter, differential double balun

coupling is the recommended input configuration. An example

is shown in Figure 6.

AD9640

VIN+

VIN –

CML

AVDD

1V p-p

49.9 Ω

523 Ω

0.1μF

499Ω

499 Ω

AD8138

Figure 6. Differential Double Balun Input Configuration

Figure 7. Differential Input Configuration Using the AD8352

An alternative to using a transformer coupled input at

frequencies in the second Nyquist zone is to use the AD8352

differential driver. An example is shown in Figure 7. See the

AD8352 datsheet for more information.

AD9640

VIN+

VIN–

49.9 Ω

CML

0.1μF

2V p-p

In any configuration, the value of the shunt capacitor, C, is

dependent on the input frequency and source impedance and

may need to be reduced or removed. Table 1 displays

recommended values to set the RC network. However, these

values will be dependant on the input signal and should only be

used as a starting guide.

Table 1 Example RC Network

Frequency Range

MHz

R series

(Ω, each)

C differential

(pF)

0-70

70-200

200-300

Open

Single-Ended Input Configuration

A single-ended input can provide adequate performance in

cost-sensitive applications. In this configuration, SFDR and

distortion performance degrade due to the large input

common-mode swing. If the source impedances on each input

are matched, there should be little effect on SNR performance.

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