Re: Differential probe

Craig Sawyers <c.sawyers@...>

could you please give me the schematic of a differential probe?
Er - there's all sorts of differential probes. There are active FET probes,
and passive ones too. The active ones are seriously difficult to get right.
Commercial active designs have to fulfill several criteria:

(a) They need to be compact
(b) They need to have a high bandwidth (>100MHz)
(c) They need to have excellent common-mode rejection over the bandwidth

Meeting these requirments leads to a complicated design, with many
adjustments to tweak the common mode. The Tek P6046 has a CMRR of 20,000:1
up to 60kHz, reducing to 2000:1 at 50MHz. The probe head has 13 miniature
transistors in it and around a dozen adjustments. The associated amplifier
has a further 12 transistors and a further 6 adjsutments. Oh - and there's
a stabilized power supply with 9 transistors.

So buying one from eBay is something of a bargain, really. I got a good
deal - I bought a brand new one (everyting still packed in the baggies, and
the Tek tag on the lead) for around $100.

Alternatively there are wide bandwidth passive differential probes, which
are a good deal easier to hand craft. These don't have the high input
impedance of the FET probes, but have very low input capacitance, which is
more important at hundreds of megahertz. Have a look at This is from a site containing lots of
interesting information on high frequency measurement techniques at


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