Op Amps grounds and input supply

[montenegromf@gmail.com]

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 n00b³

1. 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 LM358

thank you very much for any help


Marcelo

[Audioguru]

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.

[timb]

1. 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?

Can 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.

[Rerouter]

1. 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 LM358

As your dealing with electrodes connected directly to a person, i would recommend a differential connection. like so
http://hyperphysics.phy-astr.gsu.edu/hbase/electronic/ietron/diffa2.gif

This 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 rails

I 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,

[Ammar]

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!

[montenegromf@gmail.com]

Thank you for your comments. Please see below.

Nobody can teach you all about opamps in one thread. Don't you know an expert in electronics that can do what you want?
==>Reply: Thank you for you kind comments. I am not expecting this, for sure,  I post it in a beginners topic (n00b). In addition, although to try get it done by an expert is quite smart option, but who here in this forum want to escape from the challenging to make by yourself?

You do not say and might not know if your input signal is DC or is AC (what frequency?).
==>Reply: DC. No idea about 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.
==>Reply: thank you for help me make it clear. Actually I know that this LM358 has 2 opAmp, but I intend to use only one. Tips to to disable it are welcome.

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.
==>Reply: yes I did not know it, for this reason I was asking. Thank you for your clarifications. As soon I have advanced in this project I will let you know.

[montenegromf@gmail.com]

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!

Thank you very much for you suggestions. I will try double check it. 

[montenegromf@gmail.com]

1. 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 LM358

As your dealing with electrodes connected directly to a person, i would recommend a differential connection. like so
http://hyperphysics.phy-astr.gsu.edu/hbase/electronic/ietron/diffa2.gif

This 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)

==>Reply: Actually this is a electrochemical  electrode, one silver and another platinum wire immersed in chemical solutions forming a electrode. When I measure the output voltage,  it is in the range between 13 to 260 mV, which will depend of solution where the electrode is immersed.  Sorry for the confusions on nomenclature voltage vs current.

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 rails
==>Reply: hummm Thank you for answer my question.
 
I 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,

==>Reply: Thank you very much for the tips and wanings!

[montenegromf@gmail.com]

1. 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?

Can 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.
==>Reply: Thank you very much for your comments. Sorry for the confusion about the nomenclature. It is mVolts as I have measured using my multimeter. Based in your answer, it was clear for me that I need to learn these different concepts. Thank you for that. Basically, when I connect my cathode and anode to red and black pins of my multimeter, I have readings of 13 to 260 mV. When I tried to see mAmperes instead, I could not find any change
 in my multi-test.

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.

1. 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?

Can 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.
==>Reply: Thank you very much for your comments. Sorry for the confusion about the nomenclature. It is mVolts as I have measured using my multimeter. Based in your answer, it was clear for me that I need to learn these different concepts. Thank you for that. Basically, when I connect my cathode and anode to red and black pins of my multimeter, I have readings of 13 to 260 mV. When I tried to see mAmperes instead, I could not find any change
 in my multi-test.

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.
==>Reply: Thank you for your effort in try help me. I will try answer in my best. This electrode is a electrochemical electrode with 2 wires, one platinum and another silver. When I insert these wires in proper chemical solutions inside a glass pipette ( a bit more complicated that it, it is just a oversimplification to get to the point), there is exchange of electrons from the silver wire to the platinum (again to make a long story  short). When these electrons are traveling along the wires, I am able to measure the changes in voltages (in mV) in my multimeter.  These changes on mV are depending of the solutions that I insert the electrode. There is a regular mV, like a baseline exchange of electrons between the 2 wires, silver and platinum, which I measure most of times as 13 mV. When I insert the same electrode in a different solution that result in increase of rate of exchange of electrons, the mV increases, and depending of concentration of solutions, it range from 13 mV until maximum 260 mV. So, I suppose that these increasings in mV are coming from the chemical reactions and are passing trough my wire. It is a point that make it complicated for me. For example, if I have more electrons passing trough  of my wires, the current is increasing, but I am not being able to measure it in my multi-test. But I am measuring changes in voltages. Actually, if the resistance is the same (once the wires are the same) I was expecting changes in Amps and not In volts. I will search more about transimpedance amplifier.  In other words, I want to amplify this signals from mV to Volts. and why? (1) Just for leaning (2) for fun . If it works, I will be able to use this approach to amplify other electrodes that I like to play with. Here I have one of my questions. My electrode has two wires (anode and cathode), but my OpAmp has one non- inverting or one inverting input voltage, to  be amplified. How it is possible to do it? Or should I  put one wire of my electrode in the ground and another in the input of opAmp? My LM358 has 2 opAmps, I will use only one. I will try close the second by Grounding the non-inverting input of the second one.


Fill in those blanks and we can help you get a working setup.
==>Reply: I tried above.

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.
==>Reply: Thank you very much!

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.

[montenegromf@gmail.com]

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 n00b³

1. 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 LM358

thank you very much for any help


Marcelo

Finally I made it! Thank you for your help, please see the link below and just click start simulation


https://circuits.io/circuits/3654144-op-amp-amplifier-non-inverted-circuit/embed#breadboard


best regards

Marcelo

[Rerouter]

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?

[Zero999]

Don't forget the input resistance should be the same,  to minimise the effect of bias currents.

[danadak]

Here is a good tool for diff amp design -

http://www.analog.com/en/education/education-library/dh-designers-guide-to-instrumentation-amps.html


A calculator for determining voltages.


http://masteringelectronicsdesign.com/summing-amplifier-calculator-java/


Regards, Dana.