I am trying to measure the RMS value of 20MHz to 100MHz sinusoid generated from AD9951 DDS using AD8362 using single ended configuration as shown in the PIC attached.
When I feed amplitude of 3.0VP-P, I find the RMS value output VOUT measured through a DMM to be much lower than expected i.e Equation '9' VOUT = VSLP log10[rms(VIN)/VZ] or for that matter the curve shown in Figure 49. I have checked that maximum input in this range can be 2.23Vrms, still I am finding the RMS output to be lower in my case.
There after when I reduce the 1.5Vp-p or of that order, I still find the VOUT is 250mV lower than expected output for 20MHZ. This error again is not same for all frequencies and varies from 250mV to 450mV or so. I am really very confused. I do not know what to do get extremely good accuracy and better dynamic range.
We use Cin = 20nF, 20nF. DECL = 1nF, CHPF = 1nF and CLPF = 330pF.
Kindly show some light in this regards.