After 5 days of working my amplifier linearity got affected.My signal ranges from 10 millivolts to 40 millivolts.I am not getting corresponding output.please advice me.

 

Could somebody suggest me rectify the problem.I have used LT1007 Op-Amp and connected the Vout to the analog input module of the PLC.I am getting erratic readings due to varying linearity of the amplifier circuit.

 

OK, so what changed?

Is V+ still connected and of the proper value? (12V to 15v)
Is V- still connected and of the proper value? (-12v to -15v)
Has R2 become disconnected on either end, or burned open?

Short the noninverting (+) input of the opamp to ground. What is the output voltage?
Place a 100mV signal at point A in the schematic I posted. What is the output voltage?

 

Wookie:
Let me check and come back to you.

 

Wookie:

Everything seem to be Okay.I get some amount of satisfaction if I change the IC.Will it happen again? I have used 7812 & 7912 regulator ICs for Op-Amp.

 

Wookie:

Everything seem to be Okay.I get some amount of satisfaction if I change the IC.Will it happen again? I have used 7812 & 7912 regulator ICs for Op-Amp.

You may have subjected the input to voltages higher than V+ or lower than V-. This can easily happen if you are not careful with static electricity, or if there is a problem with your common grounds. This risk could be minimized by using clamping diodes, but that will introduce some non-linearity to the circuit performance due to leakage current.

What are the unregulated voltages that the 7812 and 7819 are being "fed"?

 

7812 & 7912 are fed by 24 Volts Meanwell - SMPS ( internationally renowned).I too had a doubt in ground connection.Suggest me a method to avoid this,Wookie.

You have also suggested me to use INA 118? Shall I give it a try for this application?

 

7812 & 7912 are fed by 24 Volts Meanwell - SMPS ( internationally renowned).

The 7812 and 7912 each have a dropout of 2V. Therefore, you have a differential of 20V across the V+ and V- supply terminals of the opamp. This is not good!

This could be corrected by replacing the 7912 with a 7905. You really need more like 14V to 15V on the V+ terminal. You could get there by adding a resistor from the ground terminal of the 7812 to GND. Measure the current that's flowing from the ground terminal to GND; it'll be somewhere between 5mA to 10mA. Then, R=3V/current. For example, if you measure 6mA, R=3/0.006=500 Ohms. It doesn't have to be exact for this, you just want another 2v to 3v on the V+ rail.

I too had a doubt in ground connection.Suggest me a method to avoid this,Wookie.

Make sure you have a good ground between the two before you connect to the test point. Measure it with a DVM.

You have also suggested me to use INA 114? Shall I give it a try for this application?

Sure, why not? But you'll still need to have both inputs connected.

 

I replaced the 7812 & 7912 with a SMPS which we use very often for our sensor application.Now the circuit is working well.Lessons learnt.I have ordered INA 114( not readily available in my town).

I am also yet to get LT 1010 buffer from the stores.Can OP07 be used as buffer?