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MS05000 Budget Logic Analyzer Probe Set Design

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I assembled my boards today - 4 pods and the MSO plug-in board.  I built two each for each voltage range, because why not?  It's late, only done a smoke test and the LED's come on, so far so good!   I need to give them a good wash and then take them for a ride on my MSO5074.

The 3.3v LDO was unavailable, and I thought I had some others here already.  Turns out I had the wrong package, but managed to bodge it on there anyway..


I considered adapting this design to use Vref from the MSO- providing Vcca with an amplified Vref voltage of gain = 16 should result in Vcca voltages very close to CMOS levels that are available as presets on the scope (e.g. when CMOS 1.8 is selected, Vref voltage is 112mV on my scope, after amplification Vcca would be 1.79V). The threshold levels wouldn't exactly match the indicated voltage on MSO, but appear to be close enough for the CMOS 1.2-5V ranges I'm mostly interested in). Of course, one must also limit the Vcca between 0 and max. level shifter's Vcca.

However, one important aspect to note is that SN74AXC8T245 has a propagation delay that depends on Vcc. With Vcca = 0.8V, Vccb = 3.3V the propagation delay is 0.5-35ns, which would limit it's operation at ~28MHz in the worst case.

SN74LXC8T245 which is advertised as faster than SN74AXC8T245, the LXC version appears to be worse at low voltages: at Vcca=1.2V, Vccb = 3.3V, SN74AXC8T245 has delay of 0.5-7ns, while SN74LXC8T245 has a delay of 5-22ns.

I briefly looked at high speed comparator design, but I didn't like what I saw (Rigol logic probe does not seem expensive anymore), so I'll likely take a stab at such a voltage-adjustable version and hope for the best (either of them seem to be ok for 1.8V at ~100MHz which is my immediate need).


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