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Hi I was reading on EEVBlog, some guy has reverse engineered the Logic Analyzer within the new Hantek MSO5102D MSO. I had a question about the front end part:

enter image description here

This is a 4-channel section of the front end. I understand that VT1 is used to control the threshold voltage by scaling the incoming signal through a voltage divider. But why are the capacitors needed?

Is this something to do with the inductance of the probe wires? How does this help if the user can conceivably attach any wires to the logic analyzer? - so the designer can't know what the inductance will be.

Or is this some kind of band limiting filter? If so, why is there a resistor in parallel with the capacitor?

I would be much obliged if someone could enlighten me about the design of this high speed circuit.

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It is likely that the caps are there to compensate for attenuation at high frequencies in the probes (not shown on your diagram). It probably won't be anything to do with the inductance of the probe wires and connecting any wires to the input is of course allowable but doing this will not get the best result - a proper probe needs to be used that has a tuning cap inside it that at the correct position will balance against the caps on your diagram - it's a set-up thing you do when measuring signals on oscilloscopes and similar.

Neither is it a band-limiting filter - the resistor in parallel is in fact the main natural component for the signal to go thru - the cap, as I said earlier adds a bit of high frequency compensation.

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  • \$\begingroup\$ Capacitance in the probes, or in the downstream circuitry? The downstream circuitry is bound to have some capacitance, and I would think the 4pf would serve mainly to compensate for that. \$\endgroup\$
    – supercat
    Commented Jun 3, 2013 at 17:50

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