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VCA824: Why did the Gain change so 'linear'?

user5117029
Prodigy 90 points

Part Number: VCA824

Hi everyone,

I followed the figure in datasheet of VCA824 to test the chip.

In my circuit R1 = R2 = 20 Ω, Rg = Rf = 1 kΩ.(I also wondered why R1 and R2 are not 50 Ω)

I set the Vg = -1V, 0V or ±0.9V and input a sine signal which frequency is changing between 40Mhz and 220Mhz

I powered the circuit with ±5V.

We can see that the gain in 40Mhz is about 5dB bigger than in 220Mhz and the change is linear. The character is not related to the amplitude of signal or the Vg we set.

I think the GBW of VCA824 does not cause this and I don't want the gain of chip change.

If anyone can tell me the reason of this character?

Thanks

Wong Ju

  • 0
    Guru 48395 points
    So you are looking at time domain, you should do a network analyzer sweep and see if it is rolling off with frequency, which it will do based on SSBW effects or slew limiting effect.

    The output amplifier is a current feedback, so there is no gain bandwidth product, but if you swing is high enough at the output you can lose swing amplitude going from 40MHz to 220MHz due to slew limiting.
  • 0 in reply to Michael Steffes
    Prodigy 90 points
    Hi Michael Steffes,
    Because I don't have a network analyzer, I used spectrum analyzer and signal generator to test my circuit. I changed the frequency manually and let the spectrum analyzer to record the maximum amplitude in different frequency point. The figure I uploaded is not so clear but it is at frequency domain I think.
    The VCA824 should have a 710Mhz SSBW(G = 2 V /V). I considered the slew rate, and I reduced the amplitude of input signal, but the phenomenon still remained. I am confused, can impedance mismatch or something else make it happen?

    Regards,
    Wong Ju
  • 0 in reply to user5117029
    Guru 48395 points

    The output amplifier which converts the internal signal current back into a voltage is Current feedback specified with 402ohm for that bandwidth, going to 1kohm will bandlimit it, so take Rf=Rg=402ohm and see if it doesn't work better.