4 A Note on Passband Flatness Measurements

Lastly, TI recommends that in either case, ADCs or DACs, that when performing this measurement the user uses some in-line attenuation pads connected to the input/output cable, on one or both ends. TI recommends 3dB to 6dB attenuation pads. Back calibrate this additional loss out of your measurement following the steps in this article. The main reason for additional lossy pads on the input/output cable to the analog input/output of the converter from the signal generator or spectrum analyzer is to deal with any standing waves caused by impedance mismatches.

Note that the signal generator or spectrum analyzer is expecting a good, stiff 50Ω impedance match to maximally transfer power to the load ADC (or to receive power from the DAC). The input/output impedance of the converter is never going to be a solid 50Ω across frequency, especially over a multigigahertz span and when the bandwidth begins to roll off. Standing waves accumulate and show up in measurements, causing extra ripple across the measured frequency band if not dealt with by adding additional losses to the connection path. For example, see Figure 4-1. Both of these measurements were collected in exactly the same way, except that one measurement had an attenuation pad on one end of the cable and the other measurement did not as noted in the legend in Figure 4-1.

Figure 4-1 ADC Output Response With and Without 4dB of Attenuation on the Cable