Products > Test Equipment
> 1 GHz DIY differential probes
<< < (14/35) > >>
dietert1:
Thanks for posting the probe schematics. We cannot emphasize enough the advantage of using a low capacitance active probe, even if its bandwidth is "only" 500 MHz instead of 1 GHz and even if it's not differential. When you try to make a probe for time domain measurements, you want the gain curve to level off smoothly at the upper bandwidth limit. Otherwise you will see ringing, like with audio brickwall filters. That may be a reason to prefer a discrete design since in general it will have less components. For example the proposed transistor difference stage levels off smoothly.
Anyway i would recommend to have a look at the "contemporary" TI proposals for a 2 GHz scope frontend with LMH5401 and LMH6401. Using that kind of building block is roughly how Mr. Rosenkränzer arrived at a working DIY type GHz differential probe.
Really, in 2018 we should have DSOs with builtin probe deembedding including some handy calibration scheme.
nctnico:
At some point you just can't get away with a discrete solution especially on readily available PCB materials. The parasitics will kill the circuit. Meanwhile I've updated my design with the LMH3401 to see how that works. The datasheet for the LMH3401 says that TI choose to have the feedback resistors on the chip itself to avoid the parasitic capacitance of the circuit board. Unfortunately these are 200 Ohm and for a 1:20 differential attenuation this means a differential input impedance of only 2k Ohm. It is still OK-ish because on a typical differential pair terminated with 100 Ohm it will result in an amplitude error of 5% due to loading the signal. Still seeing is believing. The simulation shows some peaking at higher frequencies (several GHz) so hopefully the loss of FR4 takes care of it.
dietert1:
I bought some ATF-35143 FETs to make buffer input stages. That works +/- 1 dB at least up to 2.9 GHz with a 51K/10K input divider made from 0805 SMD resistors. Then the parasitic capacitance of the 0805 resistor roughly compensates the FET input capacitance. Low frequency noise is less then a mV. Attached schematic describes a test setup. With smaller parts and a careful layout a 10:1 divider should compensate. I will try to drive the LMH3401EVM inputs with two of these input stages.
nctnico:
That is interesting! Are these FETs available as a matched pair? I think that will greatly improve the CMMR and DC offset matching.
Gerhard_dk4xp:
Discontinued.
No matched pairs, if that word is allowed at all in the context of FETs.
Maybe one could enforce equal currents with a mirror.

possible replacements:
<  https://www.digikey.de/product-detail/de/CE3521M4-C2/CE3521M4-C2CT-ND/6165474/?itemSeq=265991607     >

<  https://www.digikey.de/product-detail/de/CE3520K3-C1/CE3520K3-C1CT-ND/6165473/?itemSeq=265991735     >

<   https://www.digikey.de/product-detail/de/cel/CE3514M4-C2/CE3514M4-C2CT-ND/6165472     >


Also interesting:

<    https://www.digikey.de/product-detail/de/analog-devices-inc/ADL5565ACPZ-R7/ADL5565ACPZ-R7TR-ND/2773701     >

regards,
Gerhard


Navigation
Message Index
Next page
Previous page
There was an error while thanking
Thanking...

Go to full version
Powered by SMFPacks Advanced Attachments Uploader Mod