The first step is to choose the TRF1305x2 supplies. Ensure that the midsupply voltage, V_MIDSUPPLY, is between the ADC common-mode (CM) voltage and the mixer CM voltage. V_MIDSUPPLY is typically positioned closer to the ADC CM because the output CM range of the amplifier is less than the input CM range. Ensure that the dc of the signal at the input and output of the amplifier are within the valid operating common-mode voltage range. Use the MODE pin for cases where an extended range of the input CM is required.

Figure 8-7 Choosing Supply Voltages with Given Common-Mode Voltages

Figure 8-7 shows how V_MIDSUPPLY is chosen to be 0.8 V, so that the amplifier input has a CM offset from V_MIDSUPPLY of 0.8 V and output has a CM offset from V_MIDSUPPLY of 0.4 V (1.2 V – 0.8 V). The CM offsets are within the valid common-mode range of the amplifier, so the supplies of the TRF1305B2 are chosen to be V_S+ = 3.3 V (0.8 V + 2.5 V) and V_S– = −1.7 V (0.8 V – 2.5 V). Further optimization in the choice of supply is possible by selecting the input and output CM voltages for the best OIP3 performance. Section 8.2.1.3 has contour graphs that show OIP3 across input and output common-mode voltages.

The output CM is greater than the input CM; therefore, a net 6.9‑mA ((1.2 V – 0 V) / (161 Ω + 12.5 Ω)) dc current flows from the output to input through the internal feedback resistors. Depending on the choice of the passive mixer, this current can required to be sunk outside the mixer so that the bias conditions of the mixer are not disturbed. A 250‑Ω pulldown resistor connected to the INP pin to −1.7 V supply is adequate. If the 6.9‑mA dc current is sourced entirely from the amplifier, then the output headroom can be affected. Therefore, source the current externally from the supply using a pair of pullup resistors connected to the amplifier outputs. 300‑Ω pullup resistors from OUTP and OUTM to 3.3 V are adequate.

The I-channel mixer output has a 50‑Ω port and is connected to the amplifier INP pin through a small (4.7 Ω) series resistor. The INM pin is terminated to ground through a 55‑Ω resistor and to −1.7 V through a 250‑Ω resistor. This configuration makes sure that the impedance the amplifier sees at the input pins is the same at both INP and INM pins. The impedance of the mixer is close to 43 Ω and provides better than a −20‑dB return loss (theoretically). Be aware that there is some drop in the gain due to these resistor networks. Also, the values of the resistors chosen in Figure 8-6 are a good starting point; in practice, some adjustment is often needed to simultaneously meet the dc conditions and the RF performance.

At the amplifier output, 50‑Ω series resistors are used to match to the antialiasing filter with 100‑Ω differential input impedance. The filter output is connected to ADC with appropriate matching. Figure 8-6 only shows the I-channel; the Q-channel has an identical configuration.