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Making 'kind of' a differential probe

Discussion in 'Electronic Design' started by Sylvain Munaut, Apr 9, 2011.

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  1. Hi,

    I need to probe a differential signal on a board but since I'm cheap and
    don't want to buy a differential probe, I'd like to make one myself,
    plus I think I might learn a lot doing that.

    Obviously I'm not crazy, so I don't expect _anywhere_ the level of
    performance / features of a real probe ... But in the very limited scope
    of my specific signal I think it's doable.

    First a description of the signal I'm looking at (or rather, that I'd
    like to look at):

    It's the I/Q (two differential lines) of a GSM analog baseband to the RF

    - Common mode is ~ 1.3 v more or less constant during the period of
    interest, but it ramps up/down to GND before/after a burst of data.

    - The same lines are used for TX and RX, in TX mode, the variations are
    ~ 1Vpp., during RX, they are more like ~ 50mVpp.

    - The signal is GMSK 271ksym/s. So the main components are < 300 kHz

    - Those lines have a 200 ohm impedance.

    What I'd like as specs:
    - Flat gain up to 1 MHz
    - Output to scope 50 ohm
    - Hopefully something like 1Mohm / 1pF

    What would you recommend ?

    My first guess was two JFET input op amp as follower, followed by
    something like a MAX4445.
    But I'm unsure of what passives component I should use to allow the
    probe to be trimmed.

    Can I just connect the input to the jfet opamp input without anything else ?

    Thanks for any advice / schematics / part recommandation you would have.


  2. Fred Bartoli

    Fred Bartoli Guest

    Sylvain Munaut a écrit :
    Use a dual jfet opamp AD8066 as input buffers (use some protection at
    inputs), followed by an AD8130 diff amp and you'll get pretty good results.

    The 1pF part of the input impedance is somewhat optimistic.
  3. Hi John,

    Yes, that's what I'm using right now.

    The problem is that to monitor both I & Q, I need the 4 channels of my
    scope and I need to see other signals at the same time ...


  4. Hi,
    Thanks, it looks nice indeed.

    By protection at the input you mean just a series resistor, or something
    like the ESD diodes used on USB 2.0 lines ?

    Oh yes I meant 10pf :p silly me.


    Sylvain Munaut
  5. Joerg

    Joerg Guest

    That solution is too simple and doesn't cost enough ... besides, what's
    a post about electronics doing in this newsgroup? :)

    You could also buy two diff-amps of the boatanchor class at a surplus
    dealer or an auction. I did that when I needed an RF preamp for the lab
    but, of course, that felt like cheating.

    Considering that you only need 300kHz max you should be able to do this
    with a regular cheap opamp, configured as a diff amp like this:

    If you take one with modestly good offset specs and use 0.1% resistors
    you should not need to align anything. In this case I'd consider placing
    the opamp and the resistors up front in a small pod and feed it with a

    The battery can be near the scope if it makes the pod too bulky. Maybe
    you can get away with a cheap 3V coin cell if you use a modern low
    voltage opamp with a small quiescent current. If you want to be extra
    good add a circuit that shuts it off after a couple of hours or so in
    case you forget. Heck, you could even automate that fully, have it turn
    on if it sees any input wiggling and off if it didn't for xx minutes.
  6. Jamie

    Jamie Guest

    you can't use your A & B channels of your scope?

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