Opa2350 negative half of voltage - Page 1
Forum section: electronics / projects

author: Midgard
author_rank: Newbie
timestamp: August 28, 2019, 02:12:00 pm
content: I am using opa2350 as a current sensor in inverter as that combination:and when output voltage is >20 VAC i have problem with sensing current in negative sinusoidal:Positive half looks quite okay. I try it with diffrent shunt resistor value, but the problem stay always for negative voltage.Why my opamp have trouble with negative half?

author: StillTrying
author_rank: Super Contributor
timestamp: August 28, 2019, 03:35:58 pm
content: You'd see more with the scope on DC coupled.I think the input phases of the op amps are the wrong way around, they'd be better with the +ve input going to the top of the 0.04Rs to give a +ve going output with increasing 0.04R current.

author: SiliconWizard
author_rank: Super Contributor
timestamp: August 28, 2019, 03:42:12 pm
content: Quote from: StillTrying on August 28, 2019, 03:35:58 pmAgreed.

author: Midgard
author_rank: Newbie
timestamp: August 29, 2019, 02:42:20 pm
content: Thank you for answers. I switched 20k resistors with 1k and the output is a lot better.Quote from: StillTrying on August 28, 2019, 03:35:58 pmWhen I turn on DC coupling my signal is moving out of range of oscilloscope.

author: StillTrying
author_rank: Super Contributor
timestamp: August 29, 2019, 03:07:16 pm
content: "I switched 20k resistors with 1k and the output is a lot better."You reduced the gain from -20 to -1 ?We were suggesting changing the gain to +21 by swapping the 1k inputs on the 0.04Rs so that the op amp outputs rise towards 3V with more current rather than 0V.You need to know the maximum current to be measured through the 0.04Rs to set the op amp gain to give about 0-3V output, or whichever voltage range you need from the op amp output."When I turn on DC coupling my signal is moving out of range of oscilloscope."You can move the channel's 0V position to the bottom of the screen as the outputs from the op amps are all only positive(0-3V).

author: Midgard
author_rank: Newbie
timestamp: August 29, 2019, 07:04:57 pm
content: Quote from: StillTrying on August 29, 2019, 03:07:16 pmI mean i swap the resistor 20k with 1k and 1k with 20k. Sorry for not accurate explanation.Quote from: StillTrying on August 29, 2019, 03:07:16 pmHow to define maximal current on shunt resistor?

author: MarkF
author_rank: Super Contributor
timestamp: August 29, 2019, 07:22:38 pm
content: Is there a reason for using differential input configuration?I agree with others. It looks like you are inverting the signal and trying to correct for a negative output be raising the non-inverting reference.Why not just use a non-inverting configuration with a gain of 20?

author: Midgard
author_rank: Newbie
timestamp: September 02, 2019, 04:48:38 pm
content: Quote from: MarkF on August 29, 2019, 07:22:38 pmWouldn't be affecting a Bias current error?Quote from: StillTrying on August 29, 2019, 03:07:16 pmNow I have another trouble. I can't probe the output of opamp correctly. It looks like this (Pink channel):I try to solder different capacitors at top of feedback resistor but it won't help. Any ideas?

author: MarkF
author_rank: Super Contributor
timestamp: September 02, 2019, 05:07:25 pm
content: Show us your new circuit and where you are probing.Use DC coupling on the scope. Can't see the actual DC level of the signal.

author: Midgard
author_rank: Newbie
timestamp: September 03, 2019, 05:55:30 am
content: Quote from: MarkF on September 02, 2019, 05:07:25 pmMy new circuit (I probe on blue circles):Quote from: MarkF on September 02, 2019, 05:07:25 pm

author: MarkF
author_rank: Super Contributor
timestamp: September 03, 2019, 01:29:48 pm
content: You have an inverting configuration with a fixed voltage of 1.57V at the +In of each op-ampWith a gain of 1K / 20K = 0.05What are you expecting? Surely, you want to have some gain?I still think you should use my previously posted circuit.What are the voltages at RP1 and RP2? What is the max voltage you expect across these resistors?

author: MarkF
author_rank: Super Contributor
timestamp: September 03, 2019, 01:53:50 pm
content: The circuit you have is probably NOT what you think it is?Look at it redrawn.

author: Zero999
author_rank: Super Contributor
timestamp: September 03, 2019, 04:41:54 pm
content: Quote from: Midgard on September 02, 2019, 04:48:38 pmThe OPA2350 has a bias current of just 0.5pA typical, 10pA maximum and will cause an additional offset voltage of under 10nV, with an input resistance of 1k in parallel with 20k, which is tiny compared to the input offset voltage of 500µV.