Non Linearity problem (OP-AMP)

Started by June 25, 2003
 Hi all,   My design is to  control the current and it to be stable.     Current Range is  0-40A . What I made is in my PCB , I made a wider portion of copper coating  ( like shunt )  for feed back   and at two end I measure the voltage in term of mVs, and I need to amplify 0 - 10V ( 0 - 40A) . I used LM324 Op-Amp to amplify . I got the non-linear problem .   Because of very high gain needed , I put 100 Ohms input resistor ( Very low ) .   1) because of that current region can I put low resistor like this .   2)  up to 4 A current i got negative voltage O/P after postive volt O/P is coming ,  ( need to put offset volt control ) .   Is there any way to solve this problem .   Best regards Ramesh
 Hi Ramesh, I think your problem is the offset voltage of the LM324 that is around 3mV, that in your case is problem. I advise you to use the OPA277 that have a low offset voltage around 100uV. For I help you better I need to see your schematic. If you need more some thing send me your schemetic, may be I can help you.   Best Regard, Gilmar Ribeiro de Oliveira. Development Engineer. ----- Original Message ----- From: R Ramesh To: p...@yahoogroups.com Cc: Anand N Sent: Wednesday, June 25, 2003 8:03 AM Subject: [piclist] Non Linearity problem (OP-AMP)  Hi all,   My design is to  control the current and it to be stable.     Current Range is  0-40A . What I made is in my PCB , I made a wider portion of copper coating  ( like shunt )  for feed back   and at two end I measure the voltage in term of mVs, and I need to amplify 0 - 10V ( 0 - 40A) . I used LM324 Op-Amp to amplify . I got the non-linear problem .   Because of very high gain needed , I put 100 Ohms input resistor ( Very low ) .   1) because of that current region can I put low resistor like this .   2)  up to 4 A current i got negative voltage O/P after postive volt O/P is coming ,  ( need to put offset volt control ) .   Is there any way to solve this problem .   Best regards Ramesh     ">Yahoo! Terms of Service.
 Hi Ramesh, I think your problem is the offset voltage of the LM324 that is around 3mV, that in your case is problem. I advise you to use the OPA277 that have a low offset voltage around 100uV. For I help you better I need to see your schematic. If you need more some thing send me your schemetic, may be I can help you.   Best Regard, Gilmar Ribeiro de Oliveira. Development Engineer. ----- Original Message ----- From: R Ramesh To: p...@yahoogroups.com Cc: Anand N Sent: Wednesday, June 25, 2003 8:03 AM Subject: [piclist] Non Linearity problem (OP-AMP)  Hi all,   My design is to  control the current and it to be stable.     Current Range is  0-40A . What I made is in my PCB , I made a wider portion of copper coating  ( like shunt )  for feed back   and at two end I measure the voltage in term of mVs, and I need to amplify 0 - 10V ( 0 - 40A) . I used LM324 Op-Amp to amplify . I got the non-linear problem .   Because of very high gain needed , I put 100 Ohms input resistor ( Very low ) .   1) because of that current region can I put low resistor like this .   2)  up to 4 A current i got negative voltage O/P after postive volt O/P is coming ,  ( need to put offset volt control ) .   Is there any way to solve this problem .   Best regards Ramesh     ">Yahoo! Terms of Service.
 First, are you sensing on the high side (near V+) or low side (near V- )? If you use a conventional high side shunt then the easy way to get the measurement is with a Linear Technology LT1787 Precision High Side Current Sense amplifier. I use this with a 40A 0-50mV shunt on a DC motor. It has an internal gain of 8 so the 50 mV signal comes out at 400 mV. Multiplying by 5 and offsetting by 2.5V sets the signal in the center of a 0-5V A/D converter. In any event, you are measuring a differential voltage and would probably be well served to review the recent discussion on Instrumentation Amplifiers - also with a Subject of 'Linearity'. You can choose whether to build an IA with 3 Op Amps or just buy the thing already built. The Texas Instruments INA125 and INA126 look promising. The best discussion of single ended rail-to-rail Op Amps for offset and scaling is in "Op Amps For Everyone" - a free book available at www.ti.com. --- In , R Ramesh wrote: > Hi all, > > My design is to control the current and it to be stable. > > Current Range is 0-40A . What I made is in my PCB , I made a wider portion of copper coating ( like shunt ) for feed back and at two end I measure the voltage in term of mVs, and I need to amplify 0 - 10V ( 0 - 40A) . I used LM324 Op-Amp to amplify . I got the non-linear problem . > > Because of very high gain needed , I put 100 Ohms input resistor ( Very low ) . > > 1) because of that current region can I put low resistor like this . > > 2) up to 4 A current i got negative voltage O/P after postive volt O/P is coming , > ( need to put offset volt control ) . > > Is there any way to solve this problem . > > Best regards > Ramesh > > > --------------------------------- >
 Thankx a lot .     Here I attached my simple circuit to control the current and stablize it .   For ur information SHUNT is the part that I have mentioned in last mail, where I get the feedback . If u got any more Idea plz forword the circuits ,   best regards, Ramesh       Type: application/pdf

 Without a description I am not certain what you are after with the circuit. I ran it through PSPICE (Student) {FREE by the way} and got the following results with both pots set at 10K. For 0V input: With a command of 0V output is 2.2V, command of 5V yields 7.7V and a command of 10V yields 13.2V. So the loop gain for the command is 11/10 or 1.1 with an offset of 2.2V. Or, for the form y = mx + b; Vout = 1.1 * Vin + 2.2. Looks very linear to me. Now, for a command of 5V which we know will have a DC output of 7.7V and an input AC signal varying from -20mV to +20mV (40mV p-p) the output varies from 5.7V to 9.7V. Again, this is linear around the command input of 5V with a gain of 4V / 0.04 or 100. Of course the simulation is assuming ideal op amps and dual supplies (+15,-15) and simulations are not the real hardware. Nevertheless, you can apply 0 signal and expect to get the results above. Give a command of 5V with a signal source of 40mV p-p you should come close to the results of the AC analysis. As to how the MOSFETs react to the output - I didn't get into that. Perhaps the non-linearity is in the drivers, not the amplifiers. Then again, simulations are sometimes just plain wrong! --- In , R Ramesh wrote: > Thankx a lot . > Here I attached my simple circuit to control the current and stablize it . > > For ur information SHUNT is the part that I have mentioned in last mail, where I get the feedback . If u got any more Idea plz forword the circuits , > > best regards, > Ramesh > --------------------------------- >