[sdiy] Op amp clips prematurely?

nvawter at media.mit.edu nvawter at media.mit.edu
Mon Feb 3 04:40:48 CET 2014


Hey everyone - - -

Thank you all for your input!  I believe I learned a little something  
important today!  It had escaped me that not all op-amps behave as  
mathematical ideals on the inputs :)

A few people have suggested using a series resistor and gain amplifier  
to overcome the range issue.  I believe that would solve the issue,  
however, it seems it might also create an issue of having to trim  
every one of these sensors.  Therefore, I ~think~ I'd like to try  
using a different device instead, one that's input swings from rail to  
rail.  I did some sifting on DigiKey and found an inexpensive part,  
the MCP6002 (1).

The MCP6002's data sheet describes it as "rail-to-rail input and  
output swing" and it's V_CMR (Common Mode Input Range) runs from  
Vss-0.3V to Vdd+0.3V.  So I think that sounds like the proper part for  
what I'd like to do.

I'll order some of these up and let everyone know how it goes...

Thank you again for being so helpful everyone.  And if there are any  
opinions on using the MCP6002 vs. modifying the MCP607 circuit, go  
ahead and let me know.  FWIW, In the final circuit, I'll probably also  
be adding a two-pole low-pass filter with cutoffs at approx 5-20kHz.


(1) http://ww1.microchip.com/downloads/en/DeviceDoc/21733j.pdf

-Noah

p.s.  ezion: on this CPU (stm32f4), the V_AIN (Conversion Voltage  
Range) is from 0 to Vref+, where Vref+ is connected to V_DDA, which on  
my board is connected to the 3.3V supply.

p.p.s.  sorry for mis-using the reply-to functionality.  and Damien, i  
don't hate you haha.  Thanks for explaining a non-obvious feature.   
I've been using mailing lists forever, but I had no idea that was how  
it worked!

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


Quoting ezion <ezion67 at gmail.com>:

> HI Noah
> A voltage divider and matching gain from the opamp  as described by Ove
> should solve your problem.
> You might want to use a trimpot to fine tune the gain factor of the opamp.
>
> However is the ADC input on your uC rail2rail?
> This might be your next hurdle, often the build in ADC go from GND to a
> internal Vreff.
> If so variable opamp gain might solve this one too.
>
> Have fun
> Theo
>
> Groetjes Theo
>
>
> On Sun, Feb 2, 2014 at 7:33 PM, <nvawter at media.mit.edu> wrote:
>
>>
>> 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.
>>>
>>>
>>>
>>> --
>>> http://www.travisthatcher.com
>>> http://www.soundcloud.com/thevoiceofsaturn
>>> http://voiceofsaturn.blogspot.com
>>> _______________________________________________
>>> Synth-diy mailing list
>>> Synth-diy at synth-diy.org
>>> http://synth-diy.org/mailman/listinfo/synth-diy
>>>
>>>
>>
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>




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