LM324 and LM358 problem - Page 1
Forum section: electronics / beginners

author: electros6
author_rank: Regular Contributor
timestamp: September 11, 2013, 02:01:46 am
content: 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.

author: c4757p
author_rank: Super Contributor
timestamp: September 11, 2013, 02:08:55 am
content: Bad chip or you have the wrong pinout... What is "not working correctly"? What happens?

author: Rick Law
author_rank: Super Contributor
timestamp: September 11, 2013, 02:59:18 am
content: Quote from: electros6 on September 11, 2013, 02:01:46 amAh 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.

author: TerminalJack505
author_rank: Super Contributor
timestamp: September 11, 2013, 03:40:45 am
content: 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.

author: qno
author_rank: Frequent Contributor
timestamp: September 11, 2013, 09:50:47 am
content: Remember that the LM358 and the LM324 have a PNP inputstage.They work good near the ground. Not good at V+.

author: salbayeng
author_rank: Frequent Contributor
timestamp: December 25, 2016, 10:29:18 pm
content: Hi, This is a stale post, but just adding the following, in case someone finds this in a search.(a) LM358, LM324,LM2902,LM2904 are all identical internally (some have 2 others have 4 amps)They have a PNP input stage , so won't work correctly with inputs closer than 1.5v from the positive rail, it says this in the data sheet.The input offset of ~ 2mV would normally be considered excessive for current shunt measurement applicatioons.(b) There are dedicated current sense amplifiers for use on the positive railhttp://www.diodes.com/_files/datasheets/ZXCT1009.pdfis fairly common, you set the gain with a resistor referenced to ground, e.g. 1k is a gain of 1V per mV.The ZXCT1010 has a ground pin to remove the bias current from the output.(c) There are also products called "difference amplifiers" that can be used for current sensors too, some incorporate filtering , which is helpful as most power supply lines are quite noisy.

author: quickengprojects
author_rank: Newbie
timestamp: June 11, 2018, 10:59:13 am
content: Both LM324 and LM358 are different in terms of their construction i.e Lm324 comes with four inbuilt operational amplifiers while LM358 comes with only two built-in operational amplifiers. They can operate with single or split power supply.However, the execution of your project mainly depends on the absolute maximum ratings of the component you choose. You need to compare the ratings of both these amplifiers. I think different ratings are the main reason why one amplifier worked and other didn't.You can have a look athttps://www.theengineeringprojects.com/2018/02/introduction-to-lm324n.htmlfor checking ratings of lm324.

author: bd139
author_rank: Super Contributor
timestamp: June 11, 2018, 01:00:53 pm
content: Not sure why anyone uses these opamps in 2018. There are better ones for the same price!Also there are some subtle differences between earlier 324/358 and 358A’s which muddy the water.

author: ledtester
author_rank: Super Contributor
timestamp: June 11, 2018, 01:04:51 pm
content: Quote from: bd139 on June 11, 2018, 01:00:53 pmDo you have some recommendations?

author: bd139
author_rank: Super Contributor
timestamp: June 11, 2018, 01:15:29 pm
content: As always that depends on what your requirements are. Low offset, bias current, GBW etc.

author: Kleinstein
author_rank: Super Contributor
timestamp: June 11, 2018, 04:56:52 pm
content: For a low cost single supply OP the LM358 is still a good choice. More modern cheap versions like LMV358 tend to be low voltage CMOS. So better in some aspects, but only 5 or 6 V and usually more low frequency noise.

author: cellularmitosis
author_rank: Unknown
timestamp: June 12, 2018, 04:59:09 am
content: OP might not be watching this thread ~5 years on, but if this was a DIY differential amp, the error was likely a combination of the op amp Vos and the matching of the resistors used to create the differential amp.