Non linear amplifier output

Jul 08, 2010 5 Replies
R

Hi All, I am putting together a basic circuit to add 10X gain to the output of an LM34 temperature sensor (to feed a voltmeter).



So have kludged together an op-amp in the classic non-inverting input configuration. (1+R2/R1)



Single supply, 12V.



The problem is that the output shows high linearity error. The input is DC so it is not related to frequency. At low input voltages the output is lower than it should be and at higher input voltages it is higher.



I have tried two op-amps, the LMC6081 and the LM324, they both show show error. I have tried different values of R2 and R1. I have put a series resistor in the output. No change.



Rich


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J

A single-supply opamp will not work very close to ground. Check the common-mode specs.

Could the LM34 be oscillating? They love to do that. A little cable capacitance can set them off.

John

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J

Post your schematic here in LTspice format (text), or post a link to a graphic. It's not at all clear how you have it lashed up. ...Jim Thompson

| James E.Thompson, CTO | mens | | Analog Innovations, Inc. | et | | Analog/Mixed-Signal ASIC's and Discrete Systems | manus | | Phoenix, Arizona 85048 Skype: Contacts Only | | | Voice:(480)460-2350 Fax: Available upon request | Brass Rat | | E-mail Icon at http://www.analog-innovations.com | 1962 | I love to cook with wine. Sometimes I even put it in the food.
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J

On a sunny day (Thu, 8 Jul 2010 13:39:48 -0700 (PDT)) it happened rich wrote in :

The LM324 needs a resistor to gound on the output for a single supply to go all the way to zero. See page 7 National dataheet for example, they use 2k. Make sure the opamps are not oscillating. Supply decoupling etc.

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L

Also remember that the output of the temperature sensor cannot sink current to any great extent - only source it. It cannot supply bias to pnp inputs of a ground-sensing single suppy op amp, or the gain network - so load it's output before feeding the non-inverter.

To reach zero or go negative, you're going to have to float the sensor ground on something else and do a full diff gain resistor network.

RL

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R

Thanks to all for the help. Adding the output load resistor helped but the problem went away when I disconnected the voltmeter. It ran from a separate 5V regulator and was a large LED type drawing a lot of current. I am wondering whether there was a ground loop or other disturbance of ground affecting the linearity of the op-amp.

Rich

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