[sdiy] Low voltage synthesis?

Roman Sowa modular at go2.pl
Fri Jan 14 08:10:42 CET 2022


Well, obviously I meant "running at MORE than 5V"

Roman

W dniu 2022-01-14 o 08:02, Roman Sowa pisze:
> There are lots of R-R opamps running at 5V. Typicaliy at 12V, 16V, 20V 
> and even 40V.
> To name a few from my stock:
> TLV91xx
> TLV93xx - not addvertised as input R-R but has pretty low input 
> crossover error
> OPAx172
> OPAx99x
>
> Most opamp application don't even require input going higher than 
> about 1.5V from upper rail where non-R2R opamps start to get dizzy, so 
> in fact RRO (as oposed to RRIO) is all we really need in many LV 
> applications. Nearly all opamps released during last 20-30 years have 
> rail-to-rail outputs, including OPA1678.
>
> Roman
>
>
> W dniu 2022-01-13 o 22:08, Brian Willoughby pisze:
>> Most op-amps lose a couple of volts near each rail; the input and 
>> output voltages can't reach all the way to the rail voltage without 
>> lots of distortion. That's one reason to use ±12V to feed op-amps 
>> that need to accurately handle ±10V signals (of course, if your 
>> circuit regulates a ±12V power source down to ±10V for the supply 
>> rails, then the op-amps are good for less than ±8V or so in terms of 
>> signal levels). Apart from maximum supply voltage ratings, op-amps 
>> generally don't care what voltages are used, but there's always the 
>> stipulation that your signal levels will be a few volts less than the 
>> rails on either side. When the rails are as low as ±2V, there'd be no 
>> room left for (undistorted) signals.
>>
>> I've not seen any rail-to-rail op-amps that work with more than +5V, 
>> which can be treated as ±2.5V by using a virtual ground throughout 
>> the circuit, as well as capacitive coupling (*). Thus, it seems like 
>> there's a gap between the 5V rail-to-rail signal ranges and the 
>> typical 20V to 24V or more that normal op-amps can handle. Granted, 
>> there are R2R op-amps that work with less than 5V, e.g., 3.3V, but 
>> I'd be curious whether anyone knows of an R2R with more than 5V. In 
>> particular, I don't think you can manage ±5V - because there don't 
>> seem to be any 10V R2R chips - unless you're happy with signal levels 
>> within ±3V. At that point, you'd basically be better off with ±2.5V 
>> R2R than with standard op-amps operating at ±5V.
>>
>> * You can avoid capacitive coupling of signals by operating with 
>> literal bipolar supply, but that can end up with more components than 
>> a virtual ground system (like guitar pedals usually are).
>>
>> Brian Willoughby
>>
>> On Jan 13, 2022, at 09:07, Mike Bryant <mbryant at futurehorizons.com> 
>> wrote:
>>> The other issue you run against is you need to use rail-to-rail 
>>> op-amps, but most of these are specified for 5V/+-2.5V operation.   
>>> Also these opamps are often CMOS based and have what I term "input 
>>> crossover distortion" as they switch from the N to P type devices 
>>> conducting.  Negative feedback reduces this, but it's always 
>>> slightly there.  There are a few bipolar R2R opamps without this 
>>> problem - Rohm BA2510 is my favourite.
>>
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