How do I calculate the output power of the AD9739/9739A?

• Since this is a current output DAC, power is heavily dependent on the equivalent load termination the DAC output sees. The output impedance of the DAC analog outputs are 70 ohms.

The maximum peak power capability of a differential current output DAC is dependent on its peak differential ac current, IPEAK, and the equivalent load resistance it sees. Because the AD9739A includes a differential 70 Ω resistance, it is best to use a doubly terminated external output network similar to what is shown in Figure 3 below. In this case, the equivalent load (figure 4) seen by the DAC’s ac current source is 25 Ω.

Figure 3

Figure 4

If the AD9739A is programmed for IOUTFS = 20 mA, its peak ac current is 9.375 mA and its peak power delivered to the equivalent load is 2.2 mW (that is, P = (I^2)*R). Because the source and load resistance seen by the 1:1 balun are equal, this power is shared equally; therefore, the output load receives 1.1 mW or 0.4 dBm.

To calculate the rms power delivered to the load, the following must be considered:

• Peak-to-rms of the digital waveform

• Any digital backoff from digital full scale

• The DAC’s sinc response and nonideal losses in external network

For example, a reconstructed sine wave with no digital backoff ideally measures −2.6 dBm because it has a peak-to-rms ratio of 3 dB. If a typical balun loss of 0.4 dBm is included, −3 dBm of actual power can be expected in the region where the DAC’s sinc response has negligible influence. Increasing the output power is best accomplished by increasing IOUTFS, although any degradation in linearity performance must be considered acceptable for the target application.

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