Two example driving circuit configurations are shown in Figure 8-6 and Figure 8-7—one optimized for low bandwidth (low input frequencies) and the other one for high bandwidth to support higher input frequencies. Note that both of the drive circuits have been terminated by 50Ω near the ADC side. The termination is accomplished by a 25-Ω resistor from each input to the 1.5-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; good performance is obtained for high-frequency input signals. An additional termination resistor pair may be required between the two transformers, as shown in Figure 8-6, Figure 8-7, and Figure 8-8. The center point of this termination is connected to ground to improve the balance between the P and M sides. The values of the terminations between the transformers and on the secondary side must be chosen to obtain an effective 50 Ω (in the case of 50-Ω source impedance).

Figure 8-6 Drive Circuit with Low Bandwidth (for Low Input Frequencies Less Than 150 MHz)

Figure 8-7 Drive Circuit with High Bandwidth (for High Input Frequencies Greater Than 150 MHz and Less Than 270 MHz)

Figure 8-8 Drive Circuit with Very High Bandwidth (Greater than 270 MHz)

All of these examples show 1:1 transformers being used with a 50-Ω source. As explained in the Drive Circuit Requirements section, this configuration helps to present a low source impedance to absorb the sampling glitches. With a 1:4 transformer, the source impedance is 200 Ω. The higher source impedance is unable to absorb the sampling glitches effectively and can lead to degradation in performance (compared to using 1:1 transformers).

In almost all cases, either a band-pass or low-pass filter is required to obtain the desired dynamic performance, as shown in Figure 8-9. Such filters present low source impedance at the high frequencies corresponding to the sampling glitch and help avoid the performance loss with the high source impedance.

Figure 8-9 Drive Circuit with a 1:4 Transformer