AD8310 as gain detector like AD8302

Hi (Bruce?),

As I was previously working with an AD8302 gain/phase detector for a Scalar Network Analyzer, which for me has a redundant phase detection so I spent some time looking at other log amps for my V2.0 PCB. I came across the AD8307 and newer AD8310. Both appear to have a greater dynamic range than the AD8302, which is nice. But now I have a fundamental question about log amps and gain detection. (I apologize for my limited knowledge on log amps as I am actually a chemical engineer, but I am trying to learn Slight smile)

Currently, I use the AD8302 in the configuration depicted below. I am comparing the attenuation between two sinusoidal, differential signals. (One is the reference signal coming from a DDS chip and the other goes through a quartz crystal so that I can scan their resonance frequencies. Now I was wondering whether I could use the AD8310 in the same kind of configuration as I am currently using the AD8302. The datasheet only shows single ended signal inputs, but says that differential inputs are also possible. I am wondering how I could tackle this.. does this mean I would need to use two AD8310's, one to logarithmically compress the reference signal and one to compress the 'measurement' signal and then compare the two output voltages to say something about gain or attenuation? Or did I completely misunderstand the concept of a log amp? I am thinking this way since the AD8302 uses two log amps in its architecture for the gain measurement. I am looking forward to hearing from you!
Kind regards,
Rens

  • +1
    •  Analog Employees 
    on Jan 11, 2021 7:25 PM

    Hi Rens,

    It sounds like your understanding of both AD8302 and AD8310 is correct. You would need two AD8310 ICs plus a subtraction circuit to perform similar magnitude comparison that is performed by one AD8302 IC.

    The differential input capability is mentioned because it can sometimes offer improved low-level sensitivity, compared to single-ended RF drive. This is important for systems where the RF signal(s) are very weak to begin with, and best sensitivity is also a requirement.

    You probably do not want to subtract the two signals before taking the logarithmic measurement of each, because if one signal is very large and the other very small, you would loose most of the accuracy of the measurement.

    Hope that answers your question.   -Bruce H.

  • Hey Bruce,
    I did some more reading and I think I know now how the AD8310 could be used in single-ended mode. If I understand it correctly, the measurement signal coming in is referenced to the Vpos/2 at the other input. If I for example would feed a 0.5 Vpp sinusoid, the AD8310 will output a DC value that can be converted to the dBV range. I think this component will work for my SNA application and I like that it has a larger dynamic range than the AD8302. Thanks a lot!