Offset voltage question - op-amp - Page 1
Forum section: electronics / beginners

author: DuncanSteel
author_rank: Contributor
timestamp: October 22, 2017, 11:57:59 pm
content: HiI have this op-ampLM158-N(datasheet).When I measure the output voltage in voltage follower circuit I get 1,7V. My power supply battery delivers 3V & +input is set to 3V.How to find out what is the offset of the output in this condition?Currently i see voltage drop of 1,3V, but I can not find it in the datasheet.

author: Peabody
author_rank: Super Contributor
timestamp: October 23, 2017, 01:28:56 am
content: I think the problem may be in connecting the + input to the positive rail. The datasheet says the common mode input voltage includes the negative rail, but apparently not the positive rail. So your setup may produce voltage at the output pin that is "undefined".You might try connecting it to ground and see if you get any voltage at the output.However, there's an EEVBlog video on opamp offset in which Dave sets up the amp with a gain of 100 to determine how much offset voltage there is. The video includes schematics. You might want to take a look at that if you can find it.

author: wraper
author_rank: Unknown
timestamp: October 23, 2017, 01:37:46 am
content: This has nothing to do with ofset. Both input and output voltage range of x58 does not extend to positive rail. Those figures are present in the datasheet and nowhere in the datasheet it is described as rail-to-rail opamp. Power it from at least 5V and you'll see 3V on the output.

author: Audioguru
author_rank: Super Contributor
timestamp: October 23, 2017, 02:51:11 am
content: The datasheet for the LM158 says The Input Common Mode Voltage Range is 0V to V+ minus 1.5V. The maximum output voltage with no load is V+ minus 1.2V.

author: Zero999
author_rank: Super Contributor
timestamp: October 23, 2017, 08:01:58 am
content: Use an op-amp with rail-to-rail inputs and outputs, such as the TSV358, MCP602, AZV832 etc.http://ww1.microchip.com/downloads/en/DeviceDoc/21314g.pdfhttp://www.st.com/content/ccc/resource/technical/document/datasheet/4c/37/c9/ed/ee/b0/44/6d/CD00063292.pdf/files/CD00063292.pdf/jcr:content/translations/en.CD00063292.pdfhttps://www.diodes.com/assets/Datasheets/AZV831_2.pdf

author: DuncanSteel
author_rank: Contributor
timestamp: October 23, 2017, 01:28:36 pm
content: So if I power the op-amp with 5V at the positive supply I`ll get (5V - 1,5V = 3,5V) at the output ?And If I choose an op amp in the future I should look for the Voltage Range parameter?Yes I understand that rail to rail will give 2.99999 V at the output in my example with 3V supply.

author: wraper
author_rank: Unknown
timestamp: October 23, 2017, 02:06:48 pm
content: Quote from: DuncanSteel on October 23, 2017, 01:28:36 pmNo, you'll get exactly 3V at the output. Around 3.5V will bemaximumyou'll be able to get from it.QuoteNo it won't. Rail to rail opamps still are not able going exactly to the rail voltage by few tens-hundreds of mV depending on opamp model.

author: Zero999
author_rank: Super Contributor
timestamp: October 23, 2017, 05:19:03 pm
content: Quote from: DuncanSteel on October 23, 2017, 01:28:36 pmTwo parameters are important for your application:Common mode or input voltage range. This is the range of voltages the op-amp's inputs will work between. The LM358 and LM324 have an input voltage range between the op-amp's negative rail and 1.5V les than the positive rail. If one or both of the inputs are taken outside this range, the op-amp's output becomes undetermined. In some cases, phase reversal occurs, which means the inputs exchange functions: the inverting input becomes the non-inverting input and vice versa. Some op-amps have a common more range which exceeds one of the supply rails, although it's not documented, the LM358 and LM324 will work with their inputs down to 0.3V below the negative rail, but it's not guaranteed. An op-amp with a rail-to-rail input, means the input voltage range covers the entire supply voltage range.Output voltage swing. This is the range of output voltages the op-amp's output can obtain. If it's exceed, the output will just saturate at the highest or lowest voltage it can. This is always within the supply rails, unless the op-amp has a built-in charge pump, to boost the supply voltage. It's heavily dependant on the load current and often the supply voltage. A rail-to-rail output means the output will work over the full supply voltage range, although it can't quite reach each supply rail because it will have some resistance, but it's normally quite low, compared to what one would typically load an op-amp with.

author: Brutte
author_rank: Frequent Contributor
timestamp: October 23, 2017, 06:26:34 pm
content: The op-amp's properties are characterized only in some narrow range of parameters. Beyond that this is a no man's land.It won't work as follower if you power it from 1V.Or when you wire everything as described in dataseet except loading it with 1ohm - it won't output 1V outputting 1A current then.Similar with input voltage (input voltage common mode). This also has some limits.As for output voltage - again, no way to source anything when the output is above around Vcc-1.4V IIRC.etc.You can power it from 3V, no problem to make a voltage follower. However, the output works with mA currents in the range of GND+0.6V up to Vcc-1.4 so that spans from 0.6 to 1.6V (you are left with 1V span). No problems to push/pull mA currents then.You could also use the lower part GND+10mV to GND+600mV but the output sinks only ~uA here.