[sdiy] triangle core VCO comparators

[Richie Burnett]

When I was evaluating modern sot23-8 op-amps for something at work I noticed several had input clamping diodes mentioned in the datasheet. Not a problem for their intended use, but I made a mental note of it. LTC6256 is one that comes to mind, and the action of the clamping diodes is mentioned on page 13 of the datasheet. Maybe it's a Linear Technology thing.

-Richie,

Sent from my Xperia SP on O2

---- rsdio at audiobanshee.com wrote ----

>
>On Feb 26, 2016, at 11:09 PM, mskala at ansuz.sooke.bc.ca wrote:
>> On Thu, 25 Feb 2016, Richie Burnett wrote:
>>> Some op-amps have anti-parallel protection diodes between the two
>>> inputs. Not a problem in typical op-amp circuits where negative feedback
>>> keeps both inputs at almost the same potential during normal operation.
>>> But it causes problems in "comparator" service where there could be
>>> considerable voltage difference forced between the two inputs!
>> 
>> How common is this *really* in the ordinary kind of op amp today?  The
>> OP27 seems to have it.  Any others?
>
>The Linear Technologies LT1677 has ±1.4V limit, as I found out in an integrator circuit. So, it's not just comparator circuits that are sensitive to this, but integrators as well. That op-amp is super low noise, so we had to redesign the biasing to ensure both + and - were within 1.4V of each other.
>
>While searching for a replacements with similar low noise specifications, I found:
>
>LT1028/LT1128 with ±1.8V differential limit
>LT1678/LT1679 with ±1.4V differential limit
>LT1007/LT1037 with ±0.7V differential limit
>
>I wouldn't call these common or ordinary. You'd think that low noise would mean they'd have to remove the protection diodes, but I'm no op-amp design expert. As I mentioned above, we ended up redesigning the circuit to work within the differential limits because we needed the ultra low noise specifications and couldn't find a substitute that didn't have the same or similar protection diodes without getting into much higher noise specifications.
>
>Brian Willoughby
>Sound Consulting
>
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