1 Calculating maximum power delivered to load for current steering DACs

Calculations Calculate the peak-to-peak voltage at the output of the DAC –Vdac p-p = I FS * R –Where R is the total impedance seen by the DAC and I FS is the full-scale output current Find voltage at load by dividing by the sqrt of the impedance ratio –Vload p-p = Vdac p-p / sqrt(N Imp ) –Where sqrt(Nimp) = 2 for 4:1 trans, for 2:1 trans, 1 for 1:1 trans Convert peak-to-peak voltage to rms voltage –Vload rms = Vload p-p / (2 * sqrt(2) ) Calculate power DELIVERED to the load –P load = (Vload rms ) 2 / R L –R L is the impedance after the transformer, probably 50 ohms such as in the case of a spectrum analyzer hooked up to the transformer output Convert power to dBm –P dBm = 10 * log(P load / 1 mW) 2

Example 1:1 transformer Vdac p-p = (20 mA) * ((50+50) // 100 // 50) = 500 mV (25 ohm total load) Vload p-p = 500 mV / sqrt(1) = 500 mV Vload rms = 500 mV / (2 * sqrt(2) ) = mV P load = (176.8 mV) 2 / 50 = uW (spectrum analyzer = 50 ohm load) P dBm = 10 * log(625.2 uW / 1 mW) = dBm 3

Example 2:1 transformer Vdac p-p = (20 mA) * ((50+50) // (2*50)) = 1 V (50 ohm total load) Vload p-p = 1 V / sqrt(2) = mV Vload rms = mV / (2 * sqrt(2) ) = 250 mV P load = (250 mV) 2 / 50 = 1.25 mW (spectrum analyzer = 50 ohm load) P dBm = 10 * log(1.25 mW / 1 mW) = dBm 4

Example 4:1 transformer Vdac p-p = (20 mA) * (( ) // (4*50)) = 2 V (100 ohm total load) Vload p-p = 2 V / sqrt(4) = 1 V Vload rms = 1 V / (2 * sqrt(2) ) = mV P load = (353.6 mV) 2 / 50 = 2.5 mW (spectrum analyzer = 50 ohm load) P dBm = 10 * log(2.5 mW / 1 mW) = 3.98 dBm 5