Help> Shunt to ADC

Hello, I need help!
I want to monitor my shunt ex:0-50mV = 0-50Amp

I know I need to boost this mV to drive the ADC at 0-5Volts. I has build circuit with an OP Amp LM358 with 100 gain.

All work fine so I have difficulties to get the 0-50mV at input. With a voltage divider, my lower voltage is ±25mV and when I turn the pot, the voltage go up rapidly.

Anybody have solutions to monitor low voltage with an ADC, my PIC is an 16F876.

Thank's ans let me know at, mour@sympatico.ca

Eric

How much offset error can you deal with?
Where is the shunt in relation to the PIC?
Now also keep this in mind-
That op amp can't drive an output all the way to the 5v rail. And if you use a higher supply voltage on the op amp then it might drive more than 5v to the ADC pin which could toast the PIC.

There may be external ADC chips which can do a better job.

Oznog said:

How much offset error can you deal with?
Where is the shunt in relation to the PIC?
Now also keep this in mind-
That op amp can't drive an output all the way to the 5v rail. And if you use a higher supply voltage on the op amp then it might drive more than 5v to the ADC pin which could toast the PIC.

Click to expand...

Check my PIC tutorials, the analogue one uses a split supply and a 2.5V reference to overcome any supply line problems.

There may be external ADC chips which can do a better job.

Click to expand...

In what way? - over volts isn't a problem with a PIC, but is more likely to be with an external one, just feed the PIC via a current limiting resistor (2K or so), again, like my tutorial!.

Well a shunt is often better measured by something with fully differential capabilities for one. Otherwise you're using one of the shunt wires for the uC ground which can impair accuracy.

2K resistor sort of helps, but in some cases you can go straight into latch-up if you enter Sleep for example. I don't recommend it. Using the same magnitude of Vdd for the op amp and reducing Vref as you mention is a safer solution.

I give you more informations

I found into the web a PIC project... **broken link removed**

It's a GPS repeater, the GPS send NMEA code to the 16F876 and after display informations on LCD.

We have 3 ADC to monitor battery voltage, so I want to modify the C code and take the third ADC to monitor a charging system via a shunt.

I dont know nothing about C or Assembler language but I have all what we need to play with it... It's my next interest.

At this time, I dont know if any chip do what I want about cranck up mV to V. I make test with LM358 and give 100 gain, but it's semms to be instable.

I dont need extreme precision but a lot will be appreciate.

I think, this project help me to understand PIC and machine language .

Many thank's if anybody can help me with that and give some hints.

Best 73' Eric.