AD9254R703F Datasheet, PDF (12/26 Pages) Analog Devices – 14-Bit, 105 MSPS, 1.8V Analog-Tp-Digital Converter

Optimum performance is achieved by driving the AD9254 in a differential input configuration. For baseband
applications where SNR is a key parameter, differential transformer coupling is the recommended input configuration
(see Figure 6). The CML voltage can be connected to the center tap of the secondary winding of the transformer to
bias the analog input. The signal characteristics must be considered when selecting a transformer. Most RF
transformers saturate at frequencies below a few megahertz, and excessive signal power can cause core saturation,
which leads to distortion.
Figure 6. 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 AD9254. For applications where SNR is a key parameter, transformer
coupling is the recommended input. For applications where SFDR is a key parameter, differential double balun
coupling is the recommended input configuration (see Figure 7).
Figure 7. Differential Double Balun Input Configuration
As an alternative to using a transformer-coupled input at frequencies in the second Nyquist zone, the AD8351
differential driver can be used (see Figure 8)
.
Figure 8. Differential Input Configuration Using the AD8351
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 III displays recommended values to set the RC network. However,
these values are dependent on the input signal and should only be used as a starting guide.