Author Topic: limitation of opamp in the single supply differential mode  (Read 5836 times)

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Offline VanitarNordicTopic starter

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limitation of opamp in the single supply differential mode
« on: April 11, 2019, 08:31:24 am »
Hi,

I have a sensor which is connected to +12V and it shows some small variations at the output.

I have an opamp which I have configured in AC input, inverting amplifier. therefore I should supply it with the load voltage and then find the best value for + rail (by default the half of the VCC which is 6), something like the figure below, but the opamp maximum supply is 3.3V. I am bounded to use this opamp and I can not change the sensor voltage.



is there any method to fix this problem?
« Last Edit: April 11, 2019, 08:35:02 am by VanitarNordic »
 

Offline Zero999

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Re: limitation of opamp in the single supply differential mode
« Reply #1 on: April 11, 2019, 09:12:39 am »
I don't fully understand the question.

Are you asking who to use an op-amp powered from 3.3V to amplify the signal from a sensor powered by 12V?

If do, the answer is yes. You might need to add some protection diodes to the op-amp input, if they're not already built-in, to prevent spikes when the AC coupling capacitor charges, on power on.
 

Offline VanitarNordicTopic starter

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Re: limitation of opamp in the single supply differential mode
« Reply #2 on: April 11, 2019, 09:33:02 am »
I don't fully understand the question.

Are you asking who to use an op-amp powered from 3.3V to amplify the signal from a sensor powered by 12V?

If do, the answer is yes. You might need to add some protection diodes to the op-amp input, if they're not already built-in, to prevent spikes when the AC coupling capacitor charges, on power on.


No, the question is when the input is +12 referenced, the suitable voltage for the + rail could be something of course above +3.3V. in this circuit I don't see a correct output except I use a +12V opamp and adjust the + rail to see the desired wave at the output

I have already used the diode and input is AC coupled
« Last Edit: April 11, 2019, 09:34:58 am by VanitarNordic »
 

Offline Zero999

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Re: limitation of opamp in the single supply differential mode
« Reply #3 on: April 11, 2019, 11:18:58 am »
I don't fully understand the question.

Are you asking who to use an op-amp powered from 3.3V to amplify the signal from a sensor powered by 12V?

If do, the answer is yes. You might need to add some protection diodes to the op-amp input, if they're not already built-in, to prevent spikes when the AC coupling capacitor charges, on power on.


No, the question is when the input is +12 referenced, the suitable voltage for the + rail could be something of course above +3.3V. in this circuit I don't see a correct output except I use a +12V opamp and adjust the + rail to see the desired wave at the output

I have already used the diode and input is AC coupled
If the AC coupling capacitor has the correct polarity and isn't faulty, once the steady state condition has been reached, the DC voltage on the input should make no difference, to the output voltage of the op-amp. Please post a complete schematic showing the sensor, the op-amp and supply voltages clearly marked.
 

Offline StillTrying

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Re: limitation of opamp in the single supply differential mode
« Reply #4 on: April 11, 2019, 12:36:57 pm »
It's not very differential, we've been here before. :)
.  That took much longer than I thought it would.
 
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Online Ian.M

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Re: limitation of opamp in the single supply differential mode
« Reply #5 on: April 11, 2019, 06:46:36 pm »
Sim it.  Test what happens when you tie the input (to the 0.1uF cap) to +Vcc then apply a small signal to both, on top of the 12V DC bias!

It isn't differential, and it has worse than absolutely lousy common mode rejection. :(
« Last Edit: April 11, 2019, 06:48:41 pm by Ian.M »
 

Offline StillTrying

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Re: limitation of opamp in the single supply differential mode
« Reply #6 on: April 11, 2019, 07:14:55 pm »
It might be something like that, I haven't the time to play with it, inside the box is guesswork. :)
.  That took much longer than I thought it would.
 

Online Ian.M

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Re: limitation of opamp in the single supply differential mode
« Reply #7 on: April 11, 2019, 08:03:16 pm »
For the classic single OPAMP differential circuit:

to have good common mode rejection, R1/Rf must equal R2/Rg.  When capacitors are involved in the input circuit, you must consider their impedance so R1 + a series capacitor becomes Z1 and you must add a series capacitor to R2, unless the capacitive impedance is negligible with respect to R1 at the lowest frequency of interest.

If you hav capacitors in series with both R1 and R2, and split Rg into two resistors each of double its value as a potential divider between +3.3V and ground, it maintains the same Thevenin equivalent resistance for Rg, but biasses in+ to mid-rail, allowing the OPAMP to function normally.   

Input clamping is left as an exercise for the reader - if R1 and R2 are large enough your OPAMP may not even need it - see datasheet for details of what input protection it may have built-in.
 
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Offline gf

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Re: limitation of opamp in the single supply differential mode
« Reply #8 on: April 11, 2019, 09:02:20 pm »
If you hav capacitors in series with both R1 and R2, and split Rg into two resistors each of double its value as a potential divider between +3.3V and ground, it maintains the same Thevenin equivalent resistance for Rg, but biasses in+ to mid-rail, allowing the OPAMP to function normally.

Such a divider leads to a modest PSRR wrt. the +3.3V supply rail, though.
 

Offline VanitarNordicTopic starter

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Re: limitation of opamp in the single supply differential mode
« Reply #9 on: April 11, 2019, 10:22:10 pm »
Yes, I did some mistakes in resistor values. I'll modify them and see what happens
 

Online Ian.M

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Re: limitation of opamp in the single supply differential mode
« Reply #10 on: April 11, 2019, 10:33:37 pm »
Such a divider leads to a modest PSRR wrt. the +3.3V supply rail, though.
Yes.  That's the price you pay for trying to do too much with just one  single supply low voltage OPAMP.   

If the 3.3V rail (or bias supply) isn't clean, your signal is going to get FUBARed unless you can cancel out the noise in further processing, e.g if the next stage also uses a matching divider to set its reference voltage.  Otherwise, you'd do better returning the bottom end of a single Rg to a properly stabilised and filtered low impedance reference voltage that's within the rails with enough room for your amplified signal swing above and below it.
 

Offline StillTrying

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Re: limitation of opamp in the single supply differential mode
« Reply #11 on: April 12, 2019, 01:39:04 am »
If you've got the op amps +Ve input AC de-coupled to the 0V and its -Ve input AC coupled to the +12V you're just x100 amplifying the noise of the 12V supply.

Old thread. "www.eevblog.com/forum/projects/lf357-opamp-replaced-by-an-ad797-subsititude-must-work-better-but-it-dosn't!!!/msg1503595/#msg1503595"
« Last Edit: April 12, 2019, 01:42:19 am by StillTrying »
.  That took much longer than I thought it would.
 

Offline Zero999

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Re: limitation of opamp in the single supply differential mode
« Reply #12 on: April 12, 2019, 07:37:39 am »
It's not very differential, we've been here before. :)
I'm not sure it's supposed to be a differential amplifier, despite the thread title.

I thought it was just an inverting amplifier, AC coupled to a signal floating at 12VDC, with the op-amp run off 3.3V.
« Last Edit: April 12, 2019, 07:48:41 am by Zero999 »
 

Online Ian.M

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Re: limitation of opamp in the single supply differential mode
« Reply #13 on: April 12, 2019, 08:23:03 am »
Read the other thread (and  :-- to O.P. for not mentioning this is a followup  >:( ).  You've been there before!

If the O.P. has to use a ground-referenced low voltage OPAMP doing it with anything other than a gated precision differential amplifier configuration will be like listening for a mouse fart in a hurricane.  He's interested in mV or uV  level signals immediately after a 1.9V pk-pk pulse.   That's going to make life interesting (in the Chinese curse sense).
« Last Edit: April 12, 2019, 08:33:57 am by Ian.M »
 

Offline VanitarNordicTopic starter

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Re: limitation of opamp in the single supply differential mode
« Reply #14 on: April 12, 2019, 08:44:46 am »
Read the other thread (and  :-- to O.P. for not mentioning this is a followup  >:( ).  You've been there before!

If the O.P. has to use a ground-referenced low voltage OPAMP doing it with anything other than a gated precision differential amplifier configuration will be like listening for a mouse fart in a hurricane.  He's interested in mV or uV  level signals immediately after a 1.9V pk-pk pulse.   That's going to make life interesting (in the Chinese curse sense).

Thank you, Mr. Sherlock Holmes, for being a detector of my posts
I don't want to accept your insults, please do not reply to this thread, unfortunately, I can not remove this thread otherwise I can not stop people like you to talk like this.
I don't know how to deal with super professional designers here who fight with each other to prove their words instead of telling their technical opinion and leave.
 

Offline Zero999

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Re: limitation of opamp in the single supply differential mode
« Reply #15 on: April 12, 2019, 08:47:21 am »
Read the other thread (and  :-- to O.P. for not mentioning this is a followup  >:( ).  You've been there before!

If the O.P. has to use a ground-referenced low voltage OPAMP doing it with anything other than a gated precision differential amplifier configuration will be like listening for a mouse fart in a hurricane.  He's interested in mV or uV  level signals immediately after a 1.9V pk-pk pulse.   That's going to make life interesting (in the Chinese curse sense).
Which thread? I've just looked through the original poster's recent threads and can't find anything similar on the first page of hits.

I remember another recent thread about differential amplifiers but I think they might've been started by someone else.
 

Online Ian.M

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Re: limitation of opamp in the single supply differential mode
« Reply #16 on: April 12, 2019, 08:58:00 am »
StillTrying linked to the middle of it above.   Go back to page 1 and re-read.   Unless the O.P's requirements have changed in the past 11 months, the small signal, and artificial constraints (+12V referenced sensing coil and 3.3V ground referenced single OPAMP supply) will make it very difficult to get usable results.

@VanitarNordic,
If you fail to provide pertinent information like other solutions that have been tried and failed and discussed here, and that you are interested in very low level signals preceded by a high amplitude pulse, you can expect a certain amount of opprobrium.  I haven't questioned your intelligence, sanity or professional integrity, so if you feel insulted it must be because I linked a Wikipedia image to clarify the constraints on impedance ratios for a true differential OPAMP configuration.   As you have decided to put personality before engineering, I'm just going to sit back and watch  :popcorn:  Good luck - you are going to need it!
« Last Edit: April 12, 2019, 09:26:44 am by Ian.M »
 

Offline VanitarNordicTopic starter

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Re: limitation of opamp in the single supply differential mode
« Reply #17 on: April 12, 2019, 09:42:02 am »
StillTrying linked to the middle of it above.   Go back to page 1 and re-read.   Unless the O.P's requirements have changed in the past 11 months, the small signal, and artificial constraints (+12V referenced sensing coil and 3.3V ground referenced single OPAMP supply) will make it very difficult to get usable results.

@VanitarNordic,
If you fail to provide pertinent information like other solutions that have been tried and failed and discussed here, and that you are interested in very low level signals preceded by a high amplitude pulse, you can expect a certain amount of opprobrium.  I haven't questioned your intelligence, sanity or professional integrity, so if you feel insulted it must be because I linked a Wikipedia image to clarify the constraints on impedance ratios for a true differential OPAMP configuration.   As you have decided to put personality before engineering, I'm just going to sit back and watch  :popcorn:  Good luck - you are going to need it!

No, I never ever feel insulted by the technical talking or challenging the information. we learn from discussion. but please check you wasted how many hours to investigate someone else posts and make angry comments like that just because maybe the topic or content is repetitive or information is not enough.

Generally, engineers never support each other and they have the habit to ruin each other work, that's why their hard work or high working stress gets paid much less than the people in medicine who make daily mistakes in their routine job and get high salaries just because they support each other, even if their colleagues kill somebody  :(
 

Offline Zero999

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Re: limitation of opamp in the single supply differential mode
« Reply #18 on: April 12, 2019, 09:56:59 am »
StillTrying linked to the middle of it above.   Go back to page 1 and re-read.   Unless the O.P's requirements have changed in the past 11 months, the small signal, and artificial constraints (+12V referenced sensing coil and 3.3V ground referenced single OPAMP supply) will make it very difficult to get usable results.
That was nearly a year ago. I didn't think it could be related to this thread.
StillTrying linked to the middle of it above.   Go back to page 1 and re-read.   Unless the O.P's requirements have changed in the past 11 months, the small signal, and artificial constraints (+12V referenced sensing coil and 3.3V ground referenced single OPAMP supply) will make it very difficult to get usable results.

@VanitarNordic,
If you fail to provide pertinent information like other solutions that have been tried and failed and discussed here, and that you are interested in very low level signals preceded by a high amplitude pulse, you can expect a certain amount of opprobrium.  I haven't questioned your intelligence, sanity or professional integrity, so if you feel insulted it must be because I linked a Wikipedia image to clarify the constraints on impedance ratios for a true differential OPAMP configuration.   As you have decided to put personality before engineering, I'm just going to sit back and watch  :popcorn:  Good luck - you are going to need it!

No, I never ever feel insulted by the technical talking or challenging the information. we learn from discussion. but please check you wasted how many hours to investigate someone else posts and make angry comments like that just because maybe the topic or content is repetitive or information is not enough.

Generally, engineers never support each other and they have the habit to ruin each other work, that's why their hard work or high working stress gets paid much less than the people in medicine who make daily mistakes in their routine job and get high salaries just because they support each other, even if their colleagues kill somebody  :(
It's not like that where I work. Perhaps you should consider another job?

Anyway, back on topic. The op-amp should be run off the 12V rail. An AC coupled differential amplifier could be used. Awhile go Audioguru did a schematic showing the inverting and non-inverting op-amp configurations, both DC and AC coupled. Here's my version, showing the differential amplifier configuration: bottom right. Since the inputs are DC biased at more than half the supply voltage, the polarity of the input capacitors will need to be reversed.

 
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Offline gf

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Re: limitation of opamp in the single supply differential mode
« Reply #19 on: April 12, 2019, 10:51:55 am »
Anyway, back on topic. The op-amp should be run off the 12V rail. An AC coupled differential amplifier could be used.

There also do exist 3V single-supply capable instrumentation amplifier ICs with CMRR of > 80dB, which is likely better than the CMRR achievable with a differential amplifier based on a single regular opamp. In order to fit into the amp's common mode input voltage range, only AC coupling is an option then, of course. The AC ripple on the 12V rail must not exceed the amp's peak-to-peak common mode input voltage range either.

[ The first search result I found with Google is e.g. https://www.analog.com/media/en/technical-documentation/data-sheets/AD8223.pdf, but there are many others. ]
 

Offline Zero999

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Re: limitation of opamp in the single supply differential mode
« Reply #20 on: April 12, 2019, 11:06:05 am »
The link was broken. Here's the correct one.
https://www.analog.com/media/en/technical-documentation/data-sheets/AD8223.pdf

The AD8223 is a good idea. Its common mode range goes to 150mV below the negative rail, so if the peak supply ripple + signal is under that, both inputs can be biased to 0V via suitable resistors, say 1M to 10M and the input AC coupled to each input, via a capacitor.
 

Online SiliconWizard

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Re: limitation of opamp in the single supply differential mode
« Reply #21 on: April 12, 2019, 01:57:45 pm »
I've used an AD8223 for a thermocouple amplifier on a 3.3V single supply and it's great.

I suggest still checking that you're using it in its operating range. The datasheet will help, you can also use this tool: https://www.analog.com/designtools/en/diamond/#difL=-0.08&difR=0.08&difSl=-0.08&gain=100&l=-6&pr=AD8223&r=6&sl=-6&tab=1&ty=1&vn=-10&vp=10&vr=0

You may have to use a Vref > 0.
 

Offline VanitarNordicTopic starter

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Re: limitation of opamp in the single supply differential mode
« Reply #22 on: April 12, 2019, 02:34:52 pm »
Quote
Anyway, back on topic. The op-amp should be run off the 12V rail. An AC coupled differential amplifier could be used.

so you confirm my word that when the sensor is VCC referenced (+12V), then the opamp should be at least 12V and a 3.3V opamp cannot do the best job
 

Offline gf

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Re: limitation of opamp in the single supply differential mode
« Reply #23 on: April 12, 2019, 04:06:58 pm »
Quote
Anyway, back on topic. The op-amp should be run off the 12V rail. An AC coupled differential amplifier could be used.

so you confirm my word that when the sensor is VCC referenced (+12V), then the opamp should be at least 12V and a 3.3V opamp cannot do the best job

What is the peak-to-peak voltage swing at the 12V level?
a) common mode (-> basically the power supply ripple which you do not want to get amplified)
b) differential (the sensor signal itself which should be amplified)

And is AC coupling OK? You do not need DC coupling, do you?
 

Offline gf

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Re: limitation of opamp in the single supply differential mode
« Reply #24 on: April 12, 2019, 04:14:30 pm »
You may have to use a Vref > 0.

Sure, if the input is AC coupled, then it is definitively necessary to set an output offset > 0 in order to avoid clipping of the negative lobes.
 


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