LF411 Op-Amp dilemma

brad sanders radioactive at geocities.com
Wed Jan 22 22:08:08 CET 1997


>     I have yet to find a chip that works better in integrator applications 
>     than the CA3140. The TL081/2/4 is my favorite all-around op-amp, it's 
>     true, but the 3140 beats it in VCO's and VCF's. The extremely high 
>     input impedance (unrivaled, I believe) means that the low end of the 
>     integrator's range can be extended way into the sub-hertz region, yet 
>     still extend way beyond the audio range at the other end without 
>     switching in a different cap. This advantage outweighs the fact that 
>     the 3140's output must be buffered before it ever sees the end of a 
>     patchcord due to it's inability to drive standard 1K bipolar loads.

True to a degree, and I'm sure it CAN do well in the right
applications - it's just such a pain to use I'd (probably!) never
choose it for a new design, that's all.

So far as input current: there's a bevy of new devices from National,
AD and others which feature input currents several orders of magnitude
lower than the 3140. For example, the LMC66x series from National has
input impedance of 1-10 T Ohm, about a 1MHz GBP, slew rate of 0.3-1
V/uS, and input current in the 5-500 *fa* range!

Now, admittedly this is lower slew and GBP than the 3140 - but, since
we're talking audio freq. VCO, VCF, or S/H here, does it *really*
matter? 

I'm going to try the National opamps in my ladder VCF - just to see
how they "sound." I'm also going to try the Burr Brown INA113 - a x10
precision, low noise, diff input instrumentation amp-on-a-chip. A
friend has used this with good results in a phono preamp. I'm hoping,
with the OP02s, I'll have one LOW NOISE VCF (which is where most of
these others really suck, with noise voltage densities in the
15-40nV/^Hz range). 


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