Electronics > Open Source Hardware

Active probe?

(1/5) > >>

Are there any good ~1 GHz active probe open designs out there?

would simply a high-speed FET opamp work? comments, ideas?
The +/-2.5V supplies would be nice to generate from 5V USB - any ideas for a small low-noise dc-2-dc converter for that? (followed by low-noise LDOs)

David Hess:
An FET operational amplifier will work however transient response will be compromised by feedback to the inputs even if configured as a voltage follower.  I think you would actually get better performance using a medium input resistance and current feedback amplifier or even better, an operational transconductance amplifier like the OPA860 however package and circuit layout limitations will limit ultimate performance.

I think a better way is the classic JFET source follower input stage preceded by a x5 compensated divider and then bipolar emitter follower to drive a double terminated 50 ohm input yielding a total x10 attenuation.  Without any gain stage, this makes best use of the parasitic elements in printed circuit board construction.

There are some more specialized amplifier ICs which someone else may suggest but I am not familiar with them.

The main objective of an active scope probe is to reduce the 11 pF to 15 pF shunt capacitance of a normal divide by 10 passive scope probe.  Secondary objective would be to avoid  the 10x signal loss.  In order to get the capacitance down you have to have the amplifier at the probe tip end of the scope cable.

Expensive active probes have only couple tenths of pF shunt capacitance.  This is less than most common chip resistor parasitic shunt capacitance.

There are some low cost FET followers on ebay but they are capacitively coupled. 

For response to D.C. you pretty much have to use op amp.  For an op amp solution look for the highest slew rate op amps.  Keep the feedback resistors values low to prevent effects of op amp neg input node stray capacitance and layout needs to avoid nearby ground stray capacitance at the op amp - input node.  Too high feedback resistor with too much neg input node capacitance can make the op amp unstable.

The op amp output drive current must support the low value feedback resistor in parallel with 50 ohm load impedance output to scope.  Scope input needs 50 ohm termination so cable is terminated properly.  If scope does not have 50 ohm input option a BNC-T with 50 ohm terminator load will do.

The parasitics will likely kill any hope of getting some kind of bandwidth. The schematic in the first post isn't going to work at all. It starts with an 100k / 15pf RC filter.
There is a long thread about making your own (differential) 1GHz probes: https://www.eevblog.com/forum/testgear/gt-1-ghz-diy-differential-probes/ which ended up with a neat product  8)

David Hess:

--- Quote from: RCinFLA on February 15, 2020, 09:21:04 pm ---For response to D.C. you pretty much have to use op amp.
--- End quote ---

JFETs work just fine down to DC and a JFET source follower has many advantages over a series feedback amplifier.  Up to 500 MHz, the common active probe design in the past used two JFETs and two RF bipolar transistors.


[0] Message Index

[#] Next page

There was an error while thanking
Go to full version