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I have a comparator(LM393B) on a preterminated transmission line(50 ohm) which is intended to detect low and high voltages, i.e. off and active levels.

Would it be a good idea to attach a high-value series resistor R1 (i.e. 100k-1M) on the inverting input to reduce signal loading? The LM393B has BJT inputs and an open-collector output, the 1uF capacitor is per the TPA3123D2 datasheet.

Comparator circuit

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  • \$\begingroup\$ datasheet: input bias current = 3.5nA typ ; 25nA max ... no danger to overload anything. If you want to be picky, you could give it the same resistance as the other input, to reduce any differential effect from the bias current, if you think it matters to your application. unclear that it would for logic levels (and why is threshold at 0.66Vcc ?)... FInally, there may be some ESD benefit to a little bit of series resistance, as noted in the answer below. \$\endgroup\$
    – Pete W
    Feb 11 at 0:47

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No, adding a high-value resistor there is an absolutely terrible idea.

Adding the resistor doesn't change the load that the LM393 presents to the line, it merely prevents the line from driving it properly. The load current from the LM393's input will still flow just like before, but with the series resistor, it will also cause a voltage drop that makes the measurement very inaccurate. Additionally, it'll introduce a delay as it forms a RC low-pass together with the comparator's parasitic input capacitance.

A 50 ohm terminated line has quite some drive strength, so you can (and should) attach the comparator's input to it directly, or at most via a protection resistor no greater than 1k ohms.

You might also want to add a bypass capacitor (i.e. 10nF) from the comparator's non-inverting input to ground to buffer the voltage from the resistive divider.

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  • \$\begingroup\$ I am wondering, who decided to designate Ohms as ohms while Hz/kHz (or nF) the H and F are still in capital? \$\endgroup\$ Feb 10 at 23:59
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    \$\begingroup\$ @Ale..chenski As it's an SI unit, named after a person, the symbol is upper case, but the name when used as a word follows the normal rules of capitalisation, so lower case unless it starts a sentence. So as we have V and volt, we have \$\Omega\$ and ohm, Hz and hertz, H and henry. \$\endgroup\$
    – Neil_UK
    Feb 11 at 0:39
  • \$\begingroup\$ @Neil_UK, then you probably should write "A 50-ohm terminated line" instead of "50 ohm", no? \$\endgroup\$ Feb 11 at 2:19
  • \$\begingroup\$ @Ale..chenski Jonathan S should probably have written '50 \$\Omega\$ line'. I'm not sure adding a hyphen is recommended anywhere, I didn't spot it in the SI article. Anyway, Jonathan is pointing at the Moon, and we are discussing his finger. \$\endgroup\$
    – Neil_UK
    Feb 11 at 7:13

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