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Issue in LMH2120 RMS power detector output result correlation with observed input signal amplitude

Anoop Joshi
Expert 1115 points
Other Parts Discussed in Thread: LMH2120

Hi,

I'm using the LMH2120 RMS power detector part to verify the signal amplitude of a band pass filtered 60MHz sine wave. For a particular input amplitude, the output DC voltage from LMH2120 was measured as 0.2121V, from which I back calculated the Vrms value as 25.178mVrms and peak to peak voltage as 71.204mVpp. I made use of the formula given in the datasheet that Vout=Kslope*(Pin-Pint), where I used Pint=-5.5dBm(this is the value quoted for 50MHz, I believe it won't be much off for 60MHz), and got input power Pin=-18.9692dBm from which the peak to peak voltage as 71.204mVpp was back calculated for 50ohm input resistance.

But on observing the input waveform on the Keysight DS0 S 204a oscilloscope(2GHz bandwidth, internal 10bit ADC), the observed peak to peak amplitude is much higher, ie, around 115.5mVpp(scope shot attached). Can you comment on this deviation, ie, 115.5mVpp on scope vs 71.204mVpp from LMH2120


Thanks,

Anoop.

  • 0
    Expert 1115 points
    It appears the RF detector output does match the scope observation pretty well, when the clk input is connected to the scope using SMA cables, rather than probing using a passive probe. The probe noise appears to be considerable and caused the mismatch