Opamp large DC error voltage - Page 1
Forum section: electronics / projects

author: Chrische
author_rank: Newbie
timestamp: October 14, 2016, 04:50:26 pm
content: Hey,I've been working on a projekt lately and started testing some of the circuitry i designed.When i build a part of the circuit i encountered a strange error.I attached the circuit as a picture so you can see the configuration.Basically the idea is that the voltage at the output of the opamp is around 3.93V on the output when there is not "injected" another signal fromV4 or V5.When a voltage of 3.3V is applied to either V4 or V5 the output changes correspondingly.But even when i have the configuration as shown in picture there is a large DC error to what i calculated and simulated.In this configuration the voltage at the positive terminal of the opamp is 1.98V which is what i expect.As i said when no voltage is applied the output is supposed to be around 3.93V. When i measure on the circuit i build the voltage is only 3.63VThe 10V regulator supplying the device is measured to be spot on 10.03V. The resistor values are all within the spec of 1%.When i measure between the two input terminals of the opamp i measure 0.165V.Although the picture shows the device to be a TL081, the actual device is a TL082, which i assume should not matter.Anyone got any suggestions on what could cause this error? And also why the opamp does not compensate for the difference between its two input terminals?

author: ajb
author_rank: Super Contributor
timestamp: October 14, 2016, 05:08:02 pm
content: The TL082 is only spec'ed to operate to within 4V of its lower rail. The datasheet says on page 10:QuoteThe solution is to provide a negative rail (+/-15V rails are common), or to select an op amp that can operate closer to its lower rail. There are lots of op amps that will operate rail-to-rail these days, which are often attractive since they could allow you to reduce your supply voltage substantially.

author: rob77
author_rank: Super Contributor
timestamp: October 14, 2016, 06:42:28 pm
content: for your circuit use the cheap LM358 or LM2904 (dual) or LM324 (quad) - they are quite the opposite in terms of common voltage on input - they're happy at the negative rail common mode voltages on input but they are not able to reach the top rail.

author: Chrische
author_rank: Newbie
timestamp: October 15, 2016, 09:57:20 am
content: Ok, thanks for the response! I'm trying to understand what is going on.I looked at what you are saying and when i look into the datasheet i can find that the maximum peak output voltage swing for the TL082 isVOM,min = Vcc,neg + 3VVOM,max = Vcc,pos - 3VFor a +-15 supply with a 10k load.Also the graph of maximum peak output voltage vs supply voltage gives something like:+-5V supply => +-3.75V voltage swing. for a 10k loadTranslating that into what i got this meansVmin = 0V + 3.75V = +3.75VVmax =10 - 3.75V = +6.25VThat would still be within what i expect at the output (3.93V). So now i am confused. Is it really the output swing that limits here or is it something else?I suspect it is the Common mode input voltage range that is causing the problem.The parameter is only given for the +-15V supply, which might change for a different supply voltage somewhat. Not sure.This parameter will be +-11V as a worst case. Given that the voltage at the V+ terminal of the opamp is only 1.96V this might be the real problem here.Because that would mean that the minimum input voltage is 4V as far as i understand.Please correct me if I'm wrong i'm really just trying to make sense of it.

author: pelud
author_rank: Regular Contributor
timestamp: October 16, 2016, 08:00:38 am
content: Your opamp might be oscillating. Try hooking the output up to an oscilloscope and see if it's a constant voltage. Also, make sure you have 0.1uF decoupling caps on the power supply rails to ground if you don't already have them.

author: rob77
author_rank: Super Contributor
timestamp: October 16, 2016, 03:07:55 pm
content: Quote from: Chrische on October 15, 2016, 09:57:20 amyes exactly , if the common mode voltage is +-11V at +-15V supply - it means it has to be 4V above the negative rail. you don't have a negative rail , so in your case it means 4V worst case... as i told you replace the opamp with one which goes down to negative rail and it will work

author: Zero999
author_rank: Super Contributor
timestamp: October 16, 2016, 04:35:14 pm
content: Yes, youve exceeded the common mode range so this is exactly what you should expect to happen. Look at the internal schematic of the TL082. It has P-channel JFETs on its inputs. If the input voltage is taken too near the negative rail, the gate diode junction will become forward biased and divert current from the op-amps internal current mirror to the input. If this still makes no sense to you, then please do some research into JFETs.