Op Amps grounds and input supply  - Page 1
Forum section: electronics / beginners

author: montenegromf@gmail.com
author_rank: Newbie
timestamp: July 09, 2016, 01:08:36 am
content: I am biomedical scientist, which means that I have absolutely no background in electronics, engineer or things related to these fields. I am trying to build a electrode, and the final output is quite low. Watching your videos about op amp (never heard before), it seems that they may fits perfectly to my needs. My electrodes generates voltages at range 13 to 260 mV and I am trying to amplify it using opAmp, using the example EEVblog #600 =>https://youtu.be/7FYHt5XviKc?list=PLvOlSehNtuHu2FviAaZaiyXwN41G4b1Lf. I bought everything, the op amp, experimental board (I am not sure if it is the name), resistors etc... Build everything. Then, the n00b questions show up:I read about the negative voltages etc, ground in the beginners topic, but was not enough once that I am n00b31. I am trying to apply from 13 to 260 mV of current to the input of opAmp (coming from my electrode), but my electrode has 2 cables (anode and cathode) and the opAmp only one input. How could I apply the input to opAmp?2. The opAmp has the negative and positive input (vcc and vee), and I would like to set at +/- 9V. Can I use a simple battery for that? or I really need to buy a power supply?3. The output of my opAmp has only one output. How can I measure my output voltage using my multimeter, once that it has 2 connectors, black and red, and the output from the opAmp has only one output?my opAmp is the LM358thank you very much for any helpMarcelo

author: Audioguru
author_rank: Super Contributor
timestamp: July 09, 2016, 01:42:40 am
content: Nobody can teach you all about opamps in one thread. Don't you know an expert in electronics that can do what you want?You do not say and might not know if your input signal is DC or is AC (what frequency?).You say you have an LM358 opamp that has only one output but you are wrong, it has TWO opamps in it. If you do not use the second opamp then it must be properly disabled.You do not know that "+/- 9V" is not one 9V battery, it is two 9V batteries. One battery provides +9V and the second battery provides -9V. 0V is the connection between the two batteries.

author: timb
author_rank: Super Contributor
timestamp: July 09, 2016, 02:52:19 am
content: Quote from: montenegromf@gmail.com on July 09, 2016, 01:08:36 amCan you explain what you mean by this? You say "13 to 260mV" but then you call it current. mV means millivolt, which is voltage. Do you mean mA (milliamps) instead? They're totally different things. If you want to convert current to voltage you need a transimpedance amplifier.Let's assume you *do* mean voltage. 13mV is really low, so I assume you want to amplify it? Is that the case? An op-amp can be used as a simple buffer (gain of 1) or as an amplifier (with gains from 2 to thousands). So to help you, we need to know what you're trying to do. Describe where the signal is coming from, the voltage range and frequency. Then we need to know what you want to do with it, where it's going, what it's driving, how much amplification you want, etc.Fill in those blanks and we can help you get a working setup.In the mean time, here is a basic op-amp circuit, powered with two 9V batteries. It's setup as a simple voltage follower, or, buffer. This means whatever voltage you apply to the + input is mirrored onto the output. This is also referred to as a non-inverting gain of 1 configuration, or simply Av+1.

author: Rerouter
author_rank: Super Contributor
timestamp: July 09, 2016, 03:04:48 am
content: Quote from: montenegromf@gmail.com on July 09, 2016, 01:08:36 amAs your dealing with electrodes connected directly to a person, i would recommend a differential connection. like sohttp://hyperphysics.phy-astr.gsu.edu/hbase/electronic/ietron/diffa2.gifThis is more so there is no easy ways for large currents to pass to the person the electrodes mount onto,Batteries are preferred in this case as it will mean its far harder to put anyone in danger (lets say your power supply was grounded, and the person touched something with voltage)You would connect your multimeter negative to the junction point between the 2 batteries, this we would call ground, and the differential circuit i showed earlier means that the signal would be referenced to it and keep things easier for you as its no where near the weirdness op amps can have when they get near there supply railsI will agree the LM358 isnt all that great (more to shoo off later posters), but in this application it should work fine,I've attached 2 sample circuits that should work for you, the first has a gain of 22, the second a gain of 20, the main difference is the second one is very high impedance meaning it should be safer to connect to someone, but at the same time may be slightly more fragile, either will work fine,

author: Ammar
author_rank: Regular Contributor
timestamp: July 09, 2016, 03:22:01 am
content: Try searching "instrumentation amplifiers". You will find they have superior noise rejection and higher input impedance. Many have gone down this path before, so see if you can find a schematic to suit your purpose. Several application notes from the likes of TI, Analog devices and Maxim also exist, detailing the traps and things to look out for.It is a very extensive topic. Good luck!

author: montenegromf@gmail.com
author_rank: Newbie
timestamp: July 09, 2016, 08:37:30 pm
content: Quote from: Audioguru on July 09, 2016, 01:42:40 am

author: montenegromf@gmail.com
author_rank: Newbie
timestamp: July 09, 2016, 08:45:21 pm
content: Quote from: Ammar on July 09, 2016, 03:22:01 amThank you very much for you suggestions. I will try double check it.

author: montenegromf@gmail.com
author_rank: Newbie
timestamp: July 09, 2016, 09:31:13 pm
content: Quote from: Rerouter on July 09, 2016, 03:04:48 am==>Reply: Thank you very much for the tips and wanings!

author: montenegromf@gmail.com
author_rank: Newbie
timestamp: July 09, 2016, 11:27:49 pm
content: Quote from: timb on July 09, 2016, 02:52:19 am

author: montenegromf@gmail.com
author_rank: Newbie
timestamp: January 08, 2017, 12:33:03 am
content: Quote from: montenegromf@gmail.com on July 09, 2016, 01:08:36 amFinally I made it! Thank you for your help, please see the link below and just click start simulationhttps://circuits.io/circuits/3654144-op-amp-amplifier-non-inverted-circuit/embed#breadboardbest regardsMarcelo

author: Rerouter
author_rank: Super Contributor
timestamp: January 08, 2017, 05:25:47 am
content: It wont actually load on my PC (thing to do with the site), but on my phone it looked like you ended up only using 2 resistors?

author: Zero999
author_rank: Super Contributor
timestamp: January 08, 2017, 02:31:29 pm
content: Don't forget the input resistance should be the same, to minimise the effect of bias currents.

author: danadak
author_rank: Super Contributor
timestamp: January 09, 2017, 11:47:37 am
content: Here is a good tool for diff amp design -http://www.analog.com/en/education/education-library/dh-designers-guide-to-instrumentation-amps.htmlA calculator for determining voltages.http://masteringelectronicsdesign.com/summing-amplifier-calculator-java/Regards, Dana.