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

[Harry Bissell]

Resistor from the non-inverting input to ground in an inverting opamp stage.

Opamps should have equal impedances on the inverting and non-inverting inputs
to cancel out voltage offsets caused by opamp input bias currents.

In some bipolar input opamps these errors can be quite large, especially with
high bias current opamps (NE5532 etc).  

For FET inputs like the TL077x, the effect is not so noticeable.

In many designs, you just don't care (a direct connection to ground is good enough).
Low cost designs usually delete these resistors (save cost and PCB real estate)

I will often include these resistors as a matter of principle if I can fit them, it makes
it easier to if I decide they are needed, or I want to hack my own design (usually I want to :^)

The 5.1V zener on the opamp output is a bad idea. It could indeed get hot if you drive it into
full conduction. You either need a series resistor, or use the zener in the opamp feedback to
clamp the cain of the opamp, rather than 'brute force' limit the output directly.

It probably never got hot because you didn't apply an input that really stressed the output
(or your opamp is wimpy and current limits (politely) into the huge zener load...)

H^) harry




----- Original Message -----
From: Justin Owen <juzowen at gmail.com>
To: synth-diy at dropmix.xs4all.nl
Sent: Thu, 14 Jul 2011 05:45:29 -0400 (EDT)
Subject: Re: [sdiy] The annual I/O impedance/protection thread! (revised)

-----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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Harry Bissell & Nora Abdullah 4eva