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Countering Voltage Offset problems with opamps question
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Rykhaard



Joined: Sep 02, 2007
Posts: 1290
Location: Canada

PostPosted: Fri Oct 10, 2008 8:52 am    Post subject:  Countering Voltage Offset problems with opamps question Reply with quote  Mark this post and the followings unread

Alrighty - I'm finalizing the design, for my resistor divider strings for my Semitone and Octave rotary switches and have run in to a previously unknown occurrence - Voltage Offset from the opamps. Let me try this diagram to illustrate the example:

+15V to
2k7 1/4 watt to
LM336Z.2-5 - 2.5 volt voltage reference to
a resistor and multiturn trimmer to drop the output to exactly 1 volt to
12 resistors around a rotary switch to provide EXACTLY 1/12th of a volt drop per rotary switch position

Measuring the voltages from the opamp output buffer, each position steps up at exactly 0.083 volts BUT ......

The voltage output at rotary switch position #1 = 0.087V and the voltage output at rotary switch position #2 = 1.004V.

Now - I understand that if I add an offset trimmer (properly set up) to provide +/- 5 millivolts to the output buffer, that I can take the offset away completely.

NOW my question - within the entire string and flow of voltage - when I have multiple opamps in the string - do I have to have a Voltage Offset Trimmer at EVERY opamp? Or can I get away with 1 at the final opamp buffer, to take care of the whole deal?

As I see it logically - there COULD be +V offsets and -V offsets combined together from each opamp. Very possibly, they'll mostly balance each other out somewhat - able to be taken care of with an Offset trimmer at the final opamp.

SHOULD I have an offset trimmer for EVERY opamp in the string, to make sure that every single step has absolutely ZERO volts offset?

My logical reasoning says that - if each opamp ADDED together to one huge offset - I could just change the related resistor values for the final trimmer, to take care of that issue.

Help - from anyone out there? I'm just about to do the PCB for the very first of the Octave rotary switches. Taking everything into account I have:

+15V to
2k2 resistor to
LM329 +6.9V voltage reference to
Resistors to drop exactly +3.9 volts to
Resistor string around a 3 position rotary switch for 1V drop at each position to
opamp buffer to
output jack - patchable to
opamp buffer to
CD4052 2 bit multiplexer to
opamp buffer to
output jack - patchable to
opamp buffer to
CD4051 3 bit multiplexer to
opamp buffer to
output jack - patchable to any input with a 100k 1% summing resistor

There - I have 5 opamps. Reasoning it all out - I SHOULD have a Voltage Offset trimmer at the following points:

- rotary switch output buffer
- CD4052 output buffer
- CD4051 output buffer

Would that be appropriate?

(The same will apply to the new PCBs for my Semitone Rotary switches. The current 4 sets of 4 steps one does NOT have ANY Voltage Offset trimmers. Though - the 2 that are completed, are almost 100% in tune - 1V/octave wise. I am just about to do THEIR 8 input CD4051 switcher PCB.)

Thanks for any help! Smile
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