1. Home
  2. Electronics Design
  3. Making 'kind of' a differential probe

Making 'kind of' a differential probe

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 mixer:

- 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 bandwidth

- 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.

Cheers,

Sylvain

Reply to
Sylvain Munaut
Loading thread data ...

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.

--
Thanks,
Fred.
Reply to
Fred Bartoli

Two ordinary 10x probes into a hi-Z scope with A-B capability. Tweak probe compensations for max CMRR.

John

Reply to
John Larkin

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 ...

Cheers,

Sylvain

Reply to
Sylvain Munaut

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.

Cheers,

Sylvain Munaut

Reply to
Sylvain Munaut

lts.

the ad8130 datasheet says 3pf/6Mohm differential isn't that all you need ?

-Lasse

Reply to
langwadt

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:

formatting link

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 battery.

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.

--
Regards, Joerg

http://www.analogconsultants.com/

"gmail" domain blocked because of excessive spam.
Use another domain or send PM.
Reply to
Joerg

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

Jamie

Reply to
Jamie

ElectronDepot website is not affiliated with any of the manufacturers or service providers discussed here. All logos and trade names are the property of their respective owners.