[sdiy] Op amp clips prematurely?

[nvawter at media.mit.edu]

Hi, all!

I'm trying to connect a membrane potentiometer (1) to a  
microcontroller (2).  Since the pot has a nominal impedance of 20k  
(measured @18k) and the input impedance of the micro is approx 10k-50k  
(depends on several factors), I decided it would be wise to buffer it  
with an op-amp.  I chose the MCP607 (3)
because it's rail-to-rail and low-cost.  BTW, I'm running my whole  
system at 3.3V.

However, I was greatly surprised when I began testing my circuit.  I  
haven't  attached it to the micro yet.  Simply looking with a scope at  
the op-amp output has shown me a serious issue (4).  While the bottom  
end of the range produces the expected one-to-one linearity, it  
appears as if there's a discontinuity where the input range 2.8V to  
3.3V all maps to 3.3V.  To make that diagram, I connected a 20k pot as  
a voltage divider to the input of the op-amp buffer circuit and that's  
the result of turning it CW then CCW.  BTW, the scale is 1 V/seg and 1  
sec/seg.

I get the same result from a different MCP607 chip (same shipment).  I  
also get the same result driving it from a 50k pot as well as from the  
membrane potentiometer, too.  The spec for the MCP607 says the  
rail-to-rail extends to about 50mV at either end of the scale, which  
would be fine for my application but this is much further than that.

So, is there something fundamental that I'm missing?  Is this the  
proper op-amp for the task?  Is there something in the datasheet that  
would indicate to me that it's not well-suited for this task?  Or  
another recommendation for an op-amp?  Thank you very much for any  
thoughts/feedback.

-Noah

(1) https://www.sparkfun.com/products/8681
(2) STM32F405
(3) http://ww1.microchip.com/downloads/en/DeviceDoc/11177f.pdf
(4) https://plus.google.com/102831166015539659139/posts/1yXnA661VFE

-- 
http://diydsp.blogspot.com has wild and wonderful digital music instruments


Quoting Travis Thatcher <recompas at gmail.com>:

> I recently installed a Highly Liquid UMR2 in my Juno 6 to get some
> midi control. At first it seemed like everything was working but I
> then noticed that when I send chords or a lot of midi data sometimes
> notes will double trigger or random other notes will be triggered. I
> chalked this up to an issues of compatibility but I found that when I
> removed the UMR2 I still had some notes trigger periodically when I
> released the keys.  Has anyone seen this problem before? I cleaned the
> bus bar.  The arp triggers notes without any problem.
>
>
>
> --
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