The OPA695 can be used as the output stage for moderate output power arbitrary waveform driver applications. Driving out through a series 50-Ω matching resistor into a 50-Ω matched load allows up to a 4.0-V_PP swing at the matched load (15 dBm) when operating the OPA695 on a ±5-V power supply. This level of power is available for gains of either ±8 V/V with a flat response through 100 MHz. When interfacing directly from a complementary current output DAC, consider the circuit of Figure 7-7, modified for the peak output currents of the particular DAC being considered. Where purely ac-coupled output signals are required from a complementary current output DAC, consider a push-pull output stage using the circuit of Figure 7-7. The resistor values here have been calculated for a 20-mA peak output current DAC, which produces up to a 5-V_PP swing at the matched load (18 dBm). This approach gives higher power at the load, with lower 2nd-harmonic distortion.

For a 20-mA peak output current DAC, the midscale current of 10 mA gives a 2-V dc output common-mode operating voltage, due to the 200-Ω resistor to ground at the outputs. The total ac impedance at each output is 50 Ω, giving a ±0.5-V swing around this 2-V common-mode voltage for the DAC. These resistors also act as a current divider, sending 75% of the DAC output current through the feedback resistor (464 Ω). The blocking capacitor references the OPA695 output voltage to ground, and turns the unipolar DAC output current into a bipolar swing of 0.75 × 20 mA × 464 Ω = 7 V_PP at each amplifier output. Each output is exactly 180° out-of-phase from the other, producing double 7 V_PP into the matching resistors. To limit the peak output current and improve distortion, the circuit of Figure 7-7 is set up with a 1.4:1 step-down transformer. This reflects the 50-Ω load to be 100 Ω at the primary side of the transformer. For the maximum 14-V_PP swing across the outputs of the two amplifiers, the matching resistors drop this to 7 V_PP at the input of the transformer, then down to 5-V_PP maximum at the 50-Ω load at the output of the transformer. This step-down approach reduces the peak output current to 14 V_P/(200 Ω) = 70 mA.

Figure 7-7 High Power, Wideband AC-Coupled Arbitrary Waveform Driver