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AD6643 Datasheet(PDF) 20 Page - Analog Devices |
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AD6643 Datasheet(HTML) 20 Page - Analog Devices |
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20 / 36 page  AD6643 Rev. 0 | Page 20 of 36 decoupling capacitor close to the VCM pin to minimize series resistance and inductance between the device and this capacitor. Differential Input Configurations Optimum performance is achieved by driving the AD6643 in a differential input configuration. For baseband applications, the AD8138, ADA4937-2, ADA4930-2, 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 AD6643 (see Figure 28), and the driver can be configured in a Sallen-Key filter topology to provide band limiting of the input signal. 76.8Ω 120Ω 0.1µF 0.1µF 200Ω 200Ω 90Ω VIN AVDD 33Ω 33Ω 33Ω 15Ω 15Ω 5pF 15pF 15pF ADC VIN– VIN+ VCM ADA4930-2 Figure 28. Differential Input Configuration Using the ADA4930-2 For baseband applications where SNR is a key parameter, differential transformer coupling is the recommended input configuration, as shown in Figure 29. 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 R1 R1 C1 ADC VIN+ VIN– VCM C2 R2 R3 R2 C2 R3 0.1µF 33Ω Figure 29. 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 AD6643. For applications where SNR is a key parameter, differential double balun coupling is the recommended input configuration (see Figure 30). 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. 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 lists recommended values to set the RC network for different input frequency ranges. However, because these values are dependent on the input signal and bandwidth, they are to be used as a starting guide only. Note that the values given in Table 10 are for each R1, R2, C2, and R3 component shown in Figure 29 and Figure 30. Table 10. Example RC Network Frequency Range (MHz) R1 Series (Ω) C1 Differential (pF) R2 Series (Ω) C2 Shunt (pF) R3 Shunt (Ω) 0 to 100 33 8.2 0 15 49.9 100 to 300 15 3.9 0 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 amplifiers (DVGAs) provide good performance for driving the AD6643. Figure 31 shows an example of the AD8376 driving the AD6643 through a band-pass antialiasing filter. ADC R1 0.1µF 0.1µF 2V p-p VIN+ VIN– VCM C1 C2 R1 R2 R2 0.1µF S 0.1µF C2 33Ω 33Ω S PA P R3 R3 0.1µF 33Ω Figure 30. Differential Double Balun Input Configuration |
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