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# AD9781 DAC Output Power vs. Nominal Power Figure

Category: Datasheet/Specs

The AD9781 has a FS current level of 20.2mA nominally. Assuming I am driving a 50Ω load on the output, for normal mode operation I would assume the power level at 500MSPS is IFS²*RL=Pout or that the RMS voltage can be found with the same load and current. These calculations result in an output power level around 10dBm. When going through the data sheet, there is Nominal Power figure on page 15 (inserted below). I'm unsure what exactly the amplitude in the figure below is referencing. Is it referencing nominal output power of the DAC, and if so what is the impedance of the load and am I calculating output power correctly? If not, what else would the nominal power in the fundamental be referencing besides the fundamental harmonic output power? ## Top Replies

Figure 38 references the nominal output power at fundamental frequency across a 50Ω load. The peak to peak voltage across 50Ω should be…

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• Hello  ,

Figure 38 references the nominal output power at fundamental frequency across a 50Ω load. The peak to peak voltage across 50Ω should be within the output compliance range of -/+1V.

Your computation of the output power is correct if the output current is DC however the measurement shown here is across frequency and also considers the inherent sinx/x response of the DAC on both Normal and Mix Mode.

Kind Regards,

Lorenz

So I have a better understanding of the expected DAC output power in the future, should I be using the RMS IFS to account for frequency, or how can I more accurately account for frequency when calculating power?

I was using the RMS Voltage from the output compliance range of -/+1V with the assumption this would account for frequency. This does not result in the power reflected in Figure 38, even roughly accounting for the DAC's innate LPF response (Figure 65) in Normal mode operation. Thanks again, 