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Author Topic: Req: LM324 Offset Null circuit using PIC  (Read 5056 times)
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Alienbeing
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« on: June 30, 2008, 07:53:39 19:53 »

I've been using LM324 as instrumentation amplifiers with Differential input, Unity Gain for measurements of full bridge and half bridge sensors such as accelerometers for example. Offset nulling of the amplifiers have been done with 10 turn potentiometers.

Has anyone here ever achieved performing this nulling of offset voltages in an automated way? Such as using a PIC to perform the function rather then using a pot. I searched the net for awhile and can't seem to located any thing that would do the job.

The amplifiers are fed into a National Instruments data acquisiton board and are anti aliased filtered at 1650 hz prior to being sent to my Daq board.

Rather then spend hundreds or thousands of dollars for additional equipment from N.I. that can perform what I need to do, I would like to see if I can build something that will give fairly good results removing any DC offsets prior to acquiring data.

Any help is always greatly appreciated.

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Wizpic
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« Reply #1 on: June 30, 2008, 08:12:00 20:12 »

I will give it a shot if I undersatnd what your saying

what's the voltage reading coming out ?

I would say yes it can be done by using a pic aslong has you have got 0-5V coming out, If the op-amp is used to boost the voltage then I would say you need the op-amp

You could also you a voltage devider to scal the voltage down

Give us a link for the senosrs and the data board you are using then we can see what your talking about

wizpic
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Alienbeing
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« Reply #2 on: June 30, 2008, 08:30:58 20:30 »

The NI board is a PCI-MIO-16E4 set up for differential inputs. The LM324 is setup with a gain of 1 (unity gain). Inputs into the Daq cannot exceed +/-10 Volts. I already have a Endevco instrumentation amplifier model 136 for 3 of my channels into the daq. It has automatic offset nulling and adjustable gain. Really nice amp. I would like to increase the number of channels used but without having to buy another Endevco amplifier. I have alot of home built amplifiers using the LM324 and 4 pole antialise filters, but no automatic nulling. Inputs into these amps could be additional accelerometers, potentiometers (to measure angles in degrees), Linear travel pots to measure distance, or full bridge loadcells for example. I haven't used these boards in awhile but I think there is an adjustable gain on these also using another section of the LM324 just after the filter. Believe it was set for a gain of 100. These were used along time ago and were very dependable. I may still have a schematic of them available I can post later when I find it.

Typical DC offsets are encountered are usually below 100 millivolts. Temperature drift in accelerometers overtime can increase the offsets. They warm up fast when the 10 volts of excitation is applied.

Maybe a good idea would be a course nulling with a Pot, then a more fine adjustment overtime with control taken by a PIC afterwards. Maybe only monitoring and adjustment just before a trigger is received to start acquiring the data. Two triggers can be made (1) to start acquiring data in a loop so I can get some pre trigger data. (2) actual trigger to start streaming the data to the harddrive.

Alienbeing
« Last Edit: June 30, 2008, 09:01:34 21:01 by Alienbeing » Logged
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« Reply #3 on: July 12, 2008, 06:09:20 18:09 »

Maybe you can get some ideas from here on how to use digipot+opamp+PIC for offset adjustment.

http://www.maxim-ic.com/appnotes.cfm/an_pk/803
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