9.2.1.1 Transformer-Coupled Circuits

Typical applications involving transformer-coupled circuits are discussed in this section. To achieve good phase and amplitude balances at the ADC inputs, surface-mount transformers can be used (for example, for frequencies up to 300 MHz, ADT1-1WT or WBC1-1 can be used and for higher input frequencies TC1-1-13M+ can be used). When designing dc driving circuits, the ADC input impedance must be considered. Figure 149 and Figure 150 show the impedance (Z_IN = R_IN || C_IN) across the ADC input pins.

Figure 149. R_IN vs Input Frequency

Figure 150. C_IN vs Input Frequency

By using the simple drive circuit of Figure 151, uniform performance can be obtained over a wide frequency range. The buffers present at the analog inputs of the device help isolate the external drive source from the switching currents of the sampling circuit.

Figure 151. Input Drive Circuit