[sdiy] CV input op-amp circuit

[Harry]

I like to use a small resistance in series (100 ohms whatever) then a BAT54, one side to ground, the other side to the cathode of
some appropriate Zener diode to ground.  Then I use another series resistance between that node and the chip input.  You can't guarantee that the internal diodes will beat the BAT54, so the second resistor allows the BAT54 voltage to rise higher and eventually start to conduct.

Using the BAT54 isolates the zener capacitance when the diode is not conducting, and dumping the excess energy to ground is much better than
feeding it to the supply for reasons mentioned before, partial powering of the rest of the circuit.  Most DC power supplies are only good at sourcing current and can't sink current at all.

It really depends on that performance you need.  A high value resistor (100K?) could just do it all.  But if you need a low impedance
this is a good way to go.

H^) harry




----- Original Message -----
From: Richie Burnett <rburnett at richieburnett.co.uk>
To: synth-diy at synth-diy.org
Sent: Mon, 07 Dec 2020 06:56:31 -0500 (EST)
Subject: Re: [sdiy] CV input op-amp circuit

Will external Schottky diodes start to conduct before the on-die anti-static 
protection diodes?

As others have said, if you clamp external inputs to the microcontroller's 
supply rails with diodes, then be careful that you don't cause the supply 
rail to rise if the clamping current becomes significant.  The supply rail 
isn't an infinite current sink.   It can only absorb the clamping current 
either momentarily with some big decoupling capacitors, or in the 
steady-state if there is sufficient load on the supply.

-Richie,



-----Original Message----- 
From: Roman Sowa
Sent: Monday, December 7, 2020 11:37 AM
To: Mike Beauchamp ; synth-diy at synth-diy.org
Subject: Re: [sdiy] CV input op-amp circuit



W dniu 2020-12-05 o 22:42, Mike Beauchamp pisze:
>
> Regarding protecting ADC inputs, is anyone using external BAT54S schottkey 
> diodes? It seems like it might be easier to slap these in rather than rely 
> on the internal uC's diodes and the issues youve raised of sharing that 
> current if multiple pins are overloaded etc.
>

that's exactly the part I use. Among other design choices like PESD diodes.
The maximum current of input protection diodes in IC is usually 20mA,
that's clearly written in datasheets. Not that much, and with added
Schottkys the input can take much more overload without any unwanted
current wandering along tiny power traces inside the chip.

Roman
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