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> 1 GHz DIY differential probes
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nctnico:
I got a bit intrigued and rebuild the Elektor design but this time using 0402 capacitors & resistors to get better HF performance and an improved board layout which should minimise the HF peaking. This posting is an update of where I'm at now. Unfortunately my design also showed some hefty peaking so I didn't got very far. A 400MHz square wave for example:




Things get even worse for a 200MHz square wave. Time to pull out the spectrum analyser and do some sweeps. Now a differential probe should show the same signal amplitude regardless the polarity of the input signal. Interestingly reversing the polarity of the input shows two entirely different traces. In one situation it peaks and in the other it drops. The effect seems to be the strongest between 700MHz and 800 MHz. A CMMR (common mode rejection ratio) measurement also shows poor performance in the same frequency range.



This turned into a bit of a head scratcher. The simulation (using the ADA4927 model from Analog Devices) doesn't show this behaviour at all and I did not manage to find the cause in the circuit board layout. It has to be something in the amplifier which isn't in the pspice model!

At one point this graph in the ADA4927 datasheet the  caught my eye:



This shows the common mode amplification with peaking in the 750MHz region! And since the ADA4927 is not used differentially but as a common-mode amplifier this graph just explains everything. Some further research showed me that all so called fully differential amplifiers will suffer from this behaviour because the amplifier for the common mode has a poor frequency response. IMHO the fully differential amplifier are simply not suitable for use in a differential probe if the output is used in a single-ended way.
Cerebus:
I find pretty much all of nctnico's findings unsurprising. Every commercial differential probe design I've seen has frequency response trimming between both halves of the differential input (one or both sides), CMMR trim, etc. The assumption that this design makes in omitting them and expecting to get acceptable results is, IMHO, a foolish one and seems to be bourne out by measurement.
Marco:

--- Quote from: nctnico on October 14, 2018, 08:41:46 pm ---IMHO the fully differential amplifier are simply not suitable for use in a differential probe if the output is used in a single-ended way.

--- End quote ---
Need a GHz version of the AD8129.
dietert1:
Happened to find this thread after completing various measurements with a probe i bought recently from A. Rosenkränzer. In my opinion the probe is useful to record a LVDS data stream, but i would not call it a 1.9 GHz differential probe. My measurements with our HP 8560A show that ADA4927 common mode is limited to about 600 MHz if you require 3 dB accuracy. The measurement shows gain on direct (faint curve) and inverting input (bright curve).
I would consider the design incomplete. For example, he writes the probe is 1:10 while it really is 1:20. 400 mV difference on LVDS results in 20 mV in the scope (with 50 Ohm terminated input).
Anyway the concept should be valid, when elaborated carefully, e.g. with metal enclosure. Maybe a LMH3401 serves better. It is specified with a common mode bandwidth of 3.3 GHz.
Regards,
Dieter
nctnico:
The LMH3401 won't work because the minimum stable gain is >6.3 which together with the GBW product throws you back to little over 1GHz of bandwidth in an actual design. Most (if not all) of these fully differential amplifiers are designed as a front-end for fast AD converters which need a 1Vpp or 2Vpp (-ish) input signal. Also I don't think a metal enclosure will help much. All nodes in the circuit are low impedance anyway.

@Cerebus: the problem isn't in the mismatch between the inputs. I've used a symmetric PCB design and 0.1% resistors. There is no amplification so any mismatch between the inputs doesn't get multiplied. I'm convinced the problem is in the behaviour of the common mode amplifier of the ADA4927.
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