Loading on analog outputs of ADL5904

We are using 1K series, 1K shunt to ground voltage divider on each of the two analog outputs (VRMS and VCAL). This loads each output with 2K ohms.

VCAL seems to be loaded down significantly this. With the 2K load it is at 126mV, but with no load (resistors removed) it is at 717mV.

Is this normal or an indication of a failed part?

On the VRMS pin we see 308mV, and when we remove resistors it goes up to 638mV. This is inconsistent with 92 ohm internal series resistor.

  • 0
    •  Analog Employees 
    on Jun 27, 2020 1:45 AM

    Hi daver2,

    Looking through the datasheet, I can't find any spec for output impedance on those two pins. I'm wondering where you see 92 Ohms called out.

    If the device is responsive to RF input and displays the proper transfer function measured into high-Z DMM, it's probably not a failed part. Make sure you have the EPAD grounded. If any doubt, you could compare against performance on the factory eval board. Also check the output capacitor, make sure its 10nF.

    The device might have 2k internal output Rs on the VRMS pin. We cannot immediately confirm because many of us USA folks will be out of office next week, returning 7/6.


  • The 92 ohms comes from the response to another query regarding this part from another user.

    Is the value of the output cap critical?  Is this for stability or just filtering? We currently are using 22nF RF feedthrough capacitor.

    Our primary concern is whether 2K load on VRMS can damage the part.


Reply Children
  • +1
    •  Analog Employees 
    on Jul 9, 2020 12:46 AM in reply to daver2

    Hi daver2,

    Sorry about the delay getting back to you.

    The output capacitor is required for stability reasons. It's obviously going to slow the VRMS output response. Precise value is not critical. If you need the speed on this RMS output pin, you should consider another detector device choice. The digital outputs Q, Q* will not be affected and remain very fast.

    I couldn't find any precise information on the output internal series resistance for the VRMS pin.

    Hope that helps.