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Amplifying a negative millivolt signal

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#1 Joey2

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Posted 19 November 2013 - 04:36 PM

I am using an operational amplifier to amplify a positive millivolt signal by x1000 to read in one of the LabJack U12 analog inputs (single channel reading, see the attached diagram). The voltage supply I use to power the amplifier is +12V relative to the LabJack ground. Now, I would like to use this circuit to measure signals which are in the range of -1mV->-4mV. I believe that to do so I need to power the operational amplifier with something like +6.0V and -6.0V, correct? Is there a simple circuit I can use to shift my +12V power source? Is there a better way to do what I'm trying to do?

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#2 LabJack Support

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Posted 19 November 2013 - 05:56 PM

The easiest solution is to use a U6 as it has the ability to measure signals from -10V to 10V and has a built in 1000x amplifier to give you the precision you require.  Otherwise, the easiest way is to bias your input signal so that it becomes positive.  A lot of 12V supplies also have a 5V or 6V option.  Either use that to shift the voltage so that it is 6V+-4mV or add an opamp buffer shown in the google drawing link below:

#3 Joey2

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Posted 21 November 2013 - 02:16 PM

Thanks for the reply. The circuit you enclosed should work fine.
The main reason I continue to use the u12 is that I have several systems currently in operation that use u12's controlled by Java programs which include the teravation 2004 LabJackJava libraries. Java is my overwhelmingly preferred programming language because of, among other reasons, its portability. I can use the same Java code and u12 drivers running on various PC's and operating systems. I have modified the teravation jni code that interfaces the Java classes to the drivers. If I were to start using the u6 I would have to port my Java code to a newer LabJack Java library and I might have to rewrite the jni code. I haven't had the time to examine the new LabJack Java code enough to know how much work would be involved, but for now, if I can continue to use the u12 that is easiest for me.
Regards, Joe

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