| Electronics > Beginners |
| LM324 and LM358 problem |
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| electros6:
I need a differential amplifier for high side current sensing . So I build a differential amplifier using LM358 but it is not working correcting but when I built the same circuit using LM324 I got a excellent result upto 10mV resolution. Can somebody tell me what is the reason. |
| c4757p:
Bad chip or you have the wrong pinout... What is "not working correctly"? What happens? |
| Rick Law:
--- Quote from: electros6 on September 11, 2013, 02:01:46 am ---I need a differential amplifier for high side current sensing . So I build a differential amplifier using LM358 but it is not working correcting but when I built the same circuit using LM324 I got a excellent result upto 10mV resolution. Can somebody tell me what is the reason. --- End quote --- Ah ha! I have been dealing with this for just about the last moth... So it is a problem I know well even as a newbie. At 10mV, I have trouble getting two LM358 to behave the same let alone between two different chips. Each chip is different, and each OpAmp inside the chip is different. At 10mV, the small differences become big deal. An earlier attempt I had with an ATMega based volt logger, I used the LM358 and found even good size variation within the two OpAmps in the same chip. I had to calibrate the two OpAmps (on the same chip) individually. Calibration here means using software to scale it based on the specific OpAmp within the same chip. Just finishing the building the second version and this time around (rebuilding it with 4 channels) I switched over to LM2902 for the quad. Between different OpAmps (inside the same chip), I can get the 10-bit ADC reading delta of 3 to 4 counts between the OpAmps within the same chip. Between different LM2902 chips, I can get a delta of up to 6 counts. The variation between each individual OpAmps within the same chip is a lot more than 10mV. Right this moment, I am just in the process of re-calibrating each of the OpAmps after repairing a bad solder join. When properly calibrated, the 2902 (or the LM358) can handle finer division than 10mV input. At 3mV to 10mV, I can get 5-10% accuracy. At >32mV, I can get 2% accuracy. At >60mV, getting 1% accuracy is no problem. (Accuracy here means repeatable-agreement with my UT61E). Here are some screen shot's that shows the differences I did the other day (prior to the join repair which means I have to redo it...). The first two shows measuring the V-offset with two different chips and differences within the chip. The second two shows the low voltage ADC delta within the different OpAmps in the same chip. The software calibration makes different ADC count reports the same voltage. The ADC counts are not integer as you can see I am averaging multiple samples. Good luck with your quest. |
| TerminalJack505:
A schematic of how you implemented your differential amplifier might shed some light on the matter. Although, I can't think of any reason that an LM324 would work but a LM358 wouldn't. |
| qno:
Remember that the LM358 and the LM324 have a PNP inputstage. They work good near the ground. Not good at V+. |
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