AD8361 Inaccurate

Dear sir

When I use AD8361, I find that when the valid value of input voltage reaches 200 or 300 mV, the output DC voltage is no longer 7.5*Vrms, and its proportion will gradually decrease.I want to know if AD8361 has a limit on the minimum input voltage or something else is wrong.

thanks for your answer

Parents
  • +1
    •  Analog Employees 
    on Jul 20, 2021 4:30 PM

    Hello 4ever,

    AD8361 has no minimum input voltage. Test results should be similar to that shown in datasheet Figure 1 or Figure 6. 

    The device does have a broad input frequency range capability (2.5GHz), so make sure you use good quality RF signal generator and interconnects for the test.  -Bruce H. 

  • Dear Bruce_H

    I designed the circuit according to datasheet. The input end is connected with 100nF capacitor in series and 75 Ω resistor in parallel. According to the manual, the input impedance is close to 50 Ω, but the input impedance I actually measured is 75 Ω. Then there is the output problem. I use the mode of time to ground reference. First of all, the zero input will produce an output voltage of 200mV. Does that mean that my output needs to subtract 200mV to be the correct output, but even so, the scale factor is not correct. Is my input impedance not matched?

    thanks for your answer

Reply
  • Dear Bruce_H

    I designed the circuit according to datasheet. The input end is connected with 100nF capacitor in series and 75 Ω resistor in parallel. According to the manual, the input impedance is close to 50 Ω, but the input impedance I actually measured is 75 Ω. Then there is the output problem. I use the mode of time to ground reference. First of all, the zero input will produce an output voltage of 200mV. Does that mean that my output needs to subtract 200mV to be the correct output, but even so, the scale factor is not correct. Is my input impedance not matched?

    thanks for your answer

Children
  • 0
    •  Analog Employees 
    on Jul 26, 2021 6:01 PM in reply to 4ever

    Hello 4ever,

    1) AD8361 supports 3 possible offset configurations: Ground referenced, Internal referenced, or Supply referenced. See datasheet figures 36, 37, and 38. Which configuration is being used?

    2) AD8361 input resistance is approx. 225 Ohms. Also in shunt is the external 75 Ohms. The parallel combination of 75 Ohms and 225 Ohms is 56 Ohms, which should make for a very close impedance match termination at low frequencies. And for higher frequencies, the 1pF + stray capacitive reactance will bring the total down while still maintaining an acceptable impedance match.  

    The AD8361 response measurement should be close to one of the three curves shown in datasheet figure 39. 

    Notice that if configured for an output offset (the top two traces of figure 39), the offset does need to be subtracted from the output as part of the rms power calculation.   -Bruce H.