SBAS603 Data sheet

SBAS603B April 2013 – November 2020 ADS4449

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ZCR|144
- MPBGA77A

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9.2.2 Detailed Design Procedure

Two example driving circuits with a 50-Ω source impedance are shown in Figure 9-4 and Figure 9-5. The driving circuit in Figure 9-4 is optimized for input frequencies in the second Nyquist zone (centered at 185 MHz), whereas the circuit in Figure 9-5 is optimized for input frequencies in third Nyquist zone (centered at 310 MHz).

Note that both drive circuits are terminated by 50 Ω near the ADC side. This termination is accomplished with a 25-Ω resistor from each input to the 1.15-V common-mode (VCM) from the device. This architecture allows the analog inputs to be biased around the required common-mode voltage.

The mismatch in the transformer parasitic capacitance (between the windings) results in degraded even-order harmonic performance. Connecting two identical RF transformers back-to-back helps minimize this mismatch and good performance is obtained for high-frequency input signals.

Figure 9-4 Driving Circuit for a 50-Ω Source Impedance and Input Frequencies in the Second Nyquist Zone

Figure 9-5 Driving Circuit for a 50-Ω Source Impedance and Input Frequencies in the Third Nyquist Zone

TI recommends terminating the drive circuit by a 50-Ω (or lower) impedance near the ADC for best performance. However, in some applications higher impedances be required to terminate the drive circuit. Two example driving circuits with 100-Ω differential termination are shown in Figure 9-6 and Figure 9-7. In these example circuits, the 1:2 transformer (T1) is used to transform the 50-Ω source impedance into a differential 100 Ω at the input of the band-pass filter. In Figure 9-6, the parallel combination of two 68-Ω resistors and one 120-nH inductor and two 100-Ω resistors is used (100 Ω is the effective impedance in pass-band) for better performance.

Figure 9-6 Driving Circuit for a 100-Ω Source Impedance and Input Frequencies in the Second Nyquist Zone

Figure 9-7 Driving Circuit for a 100-Ω Source Impedance and Input Frequencies in the Third Nyquist Zone