Not OT: BC850 as a diode clamp?

And much lower shunt resistance.

(Responding in a new thread)

Cheers

Phil Hobbs

it works like a zener, together with the 5.1k it limits how much current can be pushed into the supply

Why would anyone care about the leakage current? This design is AC coupled. DC is not being measured. Let 'er leak!

In one hand-wired proto recently,(*) I was using an LM319N open-collector dual comparator to drive the !set and !clear inputs of one section of a 74VHC74A dflop.

The 319 was running off +-15V. By mistake, I wired the pull-up resistor on one section to +15 rather than +5. The circuit worked perfectly--I only noticed the blunder when I went to make a slight change to the ramp current source.

Abs max on both VDD and any input is +7V, which appears to be a fairly absurd underestimate of what the !clear input can comfortably manage, at least for a day or two. It didn't seem to be drawing any input current, either--at most a few dozen microamps.

Cheers

Phil Hobbs

(*) It was that highly linear ramp generator I was talking about in another thread. Swapping out the mylar cap for a polypropylene reduced the soakage tail on the ramp, but didn't eliminate it.

you ever try them xicon polystyrenes:

They're real nice, glad they haven't <knock on wood> discontinued 'em yet

The BC850 clamps in conjunction with the 22k input resistors are almost certainly there to protect the patient from excessive dc under single fault conditions. Direct current flowing through electrodes attached to the skin can cause severe burns if it is sustained for a long time - which could easily happen in some medical situations. Nobody would put 22k resistors in series with the input of a low-noise amplifier if they didn't need to. Their purpose is to limit the current that can flow when driven by the clamping voltage. Without the clamps the fault current through the electrodes would be around

John

Possibly such high-speed parts dispense with ESD diodes to reduce input capacitance?

Clifford Heath

usually so they can be 5V tolerant and/or handle input while not powered

mandag den 30. januar 2023 kl. 00.20.44 UTC+1 skrev John Walliker:

the opamps are only rated for a total supply of 5.5V which it about where the transistor would clamp so that wouldn't do much

Right, but it probably helps reduce C too.

Did this response come up in the wrong thread for everyone else too?

Lasse, your news client is weird.

CH.

Where is that stated on the schematic that Ricky originally posted?

John

So we are all trying to guess the answer based on insufficient information. Regardless of that problem, there is a requirement to limit the maximum dc (and ac, depending on frequency) currents that can flow through electrodes connected to people or animals both under normal operating conditions and in single fault conditions. See IEC 60601 for chapter and verse.

John