Non Linearity problem (OP-AMP)

Started by R Ramesh 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 <ramesh_21_in@y...> 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 <ramesh_21_in@y...> 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 > ---------------------------------
>