choosing an Op-Amp - Page 1
Forum section: electronics / beginners

author: anishkgt
author_rank: Frequent Contributor
timestamp: November 14, 2017, 11:01:13 am
content: Hi All,I am trying to learn how to use an op-amp and so far I've managed to learn it bit by bit out of experience. I have an RC filter to smooth out a PWM signal from an Arduino to control the contrast of an LCD and I've designed a buffer with the best of my knowledge.Not sure which op-amp to choose and if am not wrong it should be rail to rail - rail. my Pcb works on +5v and does not a negative supply. Would appreciate if some one could guide me here.Thanks in advance.

author: danadak
author_rank: Super Contributor
timestamp: November 14, 2017, 11:22:45 am
content: Some comments -1) This OpAmp is not RRIO, its input CM range does not include the positive rail. Vin mustbe <= 4V for Vdd = 5V.2) The output only goes to either rail if the OpAmp load current = 0.Regards, Dana.

author: anishkgt
author_rank: Frequent Contributor
timestamp: November 14, 2017, 11:31:26 am
content: Thanks Dana,That was a generic one. I was checking this out on LTSPice and i can see favorable results with the AD8031. Favaourable i mean i see a smooth curve at 100% duty cycle.Would this opamp be best suited here.

author: tszaboo
author_rank: Super Contributor
timestamp: November 14, 2017, 12:35:23 pm
content: determine:operational frequency -> GBW from thatallowed DC errors-> offset voltage and bias currentsprice of the circuitspecial featurespower supply requirements and delivery capabilitiesHead to TI.com and punch in your requirements. Use the cross reference tools for other manufacuters if you wish.

author: anishkgt
author_rank: Frequent Contributor
timestamp: November 14, 2017, 04:02:12 pm
content: Just sharing link where i found it to be useful, maybe it could help somebody.http://www.electronics-tutorials.ws/opamp/opamp_1.html

author: JanJansen
author_rank: Frequent Contributor
timestamp: November 14, 2017, 04:45:14 pm
content: Look for Microchip brand rail to rail opamps, then look to the price for the cheapest version, available 1,2 or 4 in one package.More expensive versions are faster responding you dont need that for this.

author: fcb
author_rank: Super Contributor
timestamp: November 14, 2017, 05:11:53 pm
content: Looks like your electrolytics on the filter are ass<>face.Also depends what you want to drive, and how important it is to get to 0V. I could use anything from the trusty, cheap but flawed LM358, through the excellent and fairly cheap MCP6001 family, up to an OP279G and beyond.

author: anishkgt
author_rank: Frequent Contributor
timestamp: November 14, 2017, 07:07:21 pm
content: My Load is an 20x4 LCD display and a 0 volt is not really necessary, at 1.2v the contrast is very dark so some where about 1 to 1.5v would be ok.Those are not electrolytic they are ceramic. Those symbols are US version of the cap.

author: IanMacdonald
author_rank: Frequent Contributor
timestamp: November 14, 2017, 07:53:31 pm
content: The CA3140 suggests itself here, it's able to work with a single supply, inputs and outputs near 0v.

author: anishkgt
author_rank: Frequent Contributor
timestamp: November 14, 2017, 08:28:46 pm
content: Quote from: IanMacdonald on November 14, 2017, 07:53:31 pmseems to be a bit costly.https://www.digikey.com/scripts/DkSearch/dksus.dll?Detail&itemSeq=243955443&uq=636462664722674432is something i was looking at. Would this be a perfect one ?

author: danadak
author_rank: Super Contributor
timestamp: November 14, 2017, 10:14:22 pm
content: One other issues, make sure there is a DC path from V+ input of OpAmp backto ground, eg. the PWM source must not be AC coupled into the filter input.That's to insure OpAMP bias current has a return path. No charge slowly accumulateswhich would provide an unwanted offset into OpAmp. Also if you choose to uselarger filter Rs, lower Cs, then in V- path to output insert a series R that is ~ thesame the V- sees to ground. To mitigate offset due to bias current flowing thru anR. If you pick a real low bias current part you should be able to not bother with theadditional R.Regards, Dana.

author: anishkgt
author_rank: Frequent Contributor
timestamp: November 15, 2017, 04:59:58 am
content: Thanks Dana butSounds a bit too much grasp for a beginner. I've added Resistor in series with the non-inverting to the output.QuoteIf i understand AC coupling, it is blocking DC, which would not help here.

author: fcb
author_rank: Super Contributor
timestamp: November 15, 2017, 06:42:53 pm
content: Is this for setting the LCD contrast?If so, you might not need the the opamp (they can be quite hi-z inputs), either way an LM358 would probably work fine.

author: anishkgt
author_rank: Frequent Contributor
timestamp: November 15, 2017, 06:51:20 pm
content: Yes that would be for adjusting the contrast on a 16x2 LCD. the LMV321 is a good one as it comes in the SOT23-5 which is perfect as am out of space.Thanks for the reply tho.