Logic analyzer front ends

[theatrus]

While I haven't seen many high end analyzers, I've always been interested in how the various signal levels and formats are dealt with on a logic analyzer. The cheap "put an FPGA on a board" open source devices use a simple high voltage tolerant buffer. From tear downs of upper end equipment, it appears to be chock full of custom ICs ahead of the FPGA core.

If one wanted to replicate a better front end, where would I look? A bunch of high speed comparators (I haven't found many above 150MHz)? Straight into the input gates of an FPGA?

On another note, how would one capture the timing that commercial analyzers are capable of (100s of ps)?

[T3sl4co1l]

I'd suggest a high speed comparator with all the -in's brought to one VREF (driven from a DAC), or for cheapness, a bunch of LVDS receivers instead of comparators.  The main difference is, you'll be able to find comparators with wide input voltage ranges, but LVDS probably not beyond 5V.  There might be 5V LVDS chips out there, but they'll be slower (probably not 150 meg), and still that's only 5V, not even enough to include ECL and 5V CMOS or more.

You could further add limiting/protection circuitry, that could extend the operating range, potentially at the cost of additional loading (e.g., current limiting resistors pulling on the circuit) or speed (series resistors, additional capacitance, protection diodes, etc.).

Tim

[Marco]

Why worry about a front end if you have crappy probes? The medium speed probes of the manufacturers of real LAs all seem to be passive 10x dividers so they can compensate for the capacitance of the twisted pair flying lead and the ribbon cable to the LA ... at which point yes, you need a fast comparator. Like for instance the ADCMP582 (analog has a couple slower ones too, but still faster than 150 MHz).

[theatrus]

Probing is of course a key component - I wasn't intending on excluding the probe harness from the "front end" portion.

The LMH7324 is an interesting choice here as well: http://www.ti.com/product/lmh7324 Even as a quad, and a lower price: 

I'm going to do a bit more digging into how the probe and leads are usually built.

[MathWizard]

Can a videocard be used for anything like a logic analzyer, or other test gear ? They have very fast data rates, over LVDS lines, and they run games good, and do lots of fast math. Most of that won't be needed I guess for a LA.

IDR the input voltage range for all the PCIe lines, or if they are clocked, but if suitable level shifters were used, and the data lines can be clocked right, can a GPU be hacked into a logic analyzer ? Maybe they aren't that similar. But I'm pretending any security features are no problem, and the programer/hacker has full knowledge of a GPU, and can mod it too.

I'm just picturing a bunch of Schmitt trigger type inputs on the PCIe inputs, and there's a lot of PCEe pins, to sense or send the data, and they are fast.

[jbb]

Good find with the LMH7324.

For a serious design, I would consider deploying 2 comparators per channel and setting separate ‘input low’ and ‘input high’ thresholds. Higher level parts of the instrument can then compare the two signals to look for things like poor slew rates or runt pulses (https://www.siglenteu.com/operating-tip/capture-runt-pulse-siglent-sds-oscilloscope/)