[sdiy] The annual I/O impedance/protection thread! (revised)

[Tom Wiltshire]

Just to say that I've been following this thread with interest, so it's not just for Justin's benefit.

Thanks,
Tom

On 14 Jul 2011, at 10:45, Justin Owen wrote:

> -----Original Message-----
> From: Neil Johnson [neil.johnson97 at ntlworld.com]
> 
>> Inputs
>> http://www.sdiy.org/juz/euro_input_03.pdf
> 
>>> Harry wrote:
>>> I'd suggest this... use only inverting inputs
> 
>> That's probably the best compromise, and means you can effectively drop
>> the zeners and the 100R resistors.
> 
> Yes, completely sold on going with inverting inputs as standard.
> 
>> Note that for the AC-coupled case you need to either use a non-polarised 
>> capacitor, or think about what the DC operating point of the op-amp -ve 
>> pin will be to determine which way round the polarized electrolytic goes.
> 
> Yes, Harry pointed out some factors that answered some questions I'd had about AC coupling cap choice for ages.
> I'll probably go with 1uF Mylars.
> 
>> Outputs
>> http://www.sdiy.org/juz/euro_output_03.pdf
> 
>> And for the non-inverting circuits add anti-parallel 
>> Schottky diodes across the op-amp inputs and a resistor to the +ve input.
> 
> I've added Schottky diodes based on advice from you and Harry B - it's also been suggested that I could use 1N4148s.
> Can you clarify what you mean about the resistor to the +ve input? Where to/from? Why? Value based on?
> 
>> In the AC output versions I'd just put the series protection resistor 
>> after the feedback tap, then series capacitor (watch the polarity!) and 
>> high-ish drain resistor.
> 
> You'd ditch the zeners and their associated protection resistor from the output as well?
> 
>> Summing
>> http://www.sdiy.org/juz/euro_summing_03.pdf
> 
>> 2/ R1 will appreciably load the input pot to give a non-linear control 
>> curve.  This might not be an issue for you (designers choice) but it 
>> irks me.
> 
> So how would you lay out an attenuated input for an inverting summing config or what would you suggest revising about that input with regards to component choice, value, order ,etc?
> 
>> 3/ Your manual CV pot reference voltage is negative, 
>> 4/ ...But this won't be the case for the external CV
> 
> Yes. My bad. Corrected.
> 
>> 5/ Depending on what the internal load looks like that 1N4001 could 
>> behave in a rather non-linear fashion.  And assuming enough current 
>> passes through it to turn it on, it will drop about 1V from the output 
>> of the op-amp.
> 
> Yes, agreed, but considering it will probably be feeding something along the lines of my beloved single transistor VCA - it does the job just fine. When I need a precision solution I'll use a 'proper' rectifier.
> 
>> 6/ When the 5V1 zener turns on its going to provide a nice low-impedance 
>> path to ground to pull lots of current out of the op-amp, which will 
>> overheat and possibly melt.  Not quite desirable.
> 
> I've used this config plenty of times and not even noticed any undue warmth - and I do check. Certainly never had one melt. 
> 
> Your impedance point does concern me though. Considering that the aim of the 1N4001/5V1 Zener combo is to allow an AC or negative V input to be used as a CV source, then attenuated, biased, blocked and capped so you only get e.g. 0-5V - what would you suggest for capping the upper DC limit?
> 
> Thanks - appreciated,
> 
> Justin
> 
> 
> 
> 
> 
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