Opamp with unbalanced rails

Question

I'm using this - opamp.

I'm powering the MCP6V81 off +0.5V and -5.0V supplies. So the supplies are very unbalanced. Will this cause us any issues? Such as output offset, and drift?

Also, in the absolute maximum voltage rating, it is mentioned as Vdd-Vss = 6.5V.

Is it the difference between two voltages that must not be more than 6.5V? Can Vss be lesser than 0V, say -5V as in my case? Or should Vss be 0V?

The minimum supply voltage is 2.2V. Even if Vss is -4.3V, the device condition of Vdd-Vss=6.5V is satisfied right?

you should determine your input voltage range of your Opamp!!! — Rashid, Mar 14 '23 at 11:26
Should not be an issue but using up 5.5 out of 6.5 V of absolute maximum rating (85 %) is a bit on the high side. Is this a consumer product or some high reliability military/mission critical industrial/aviation end product? — winny, Mar 14 '23 at 11:41
@winny The data sheet rates it up to 5.5V nominal. The op-amp will likely be fine with the supplies. Another thing is, can it do anything useful in a circuit with the supplies it has, as the circuit, inputs and outputs are currently unknown. — Justme, Mar 14 '23 at 12:40

Andy aka · Accepted Answer · 2023-03-14T15:35:31.490

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So the supplies are very unbalanced. Will this cause us any issues? Such as output offset, and drift?

0 volts is just a convenient concept for us as engineers. An op-amp doesn't recognize or know about 0 volts. For the op-amp you list, its inputs are good to work to either rail and the output can swing to typically 50 mV of each power rail on a 1 kΩ load. So, providing you don't expect something better than this, it should be fine.

Is it the difference between two voltages that must not be more than 6.5V?

6.5 volts is the absolute maximum between \$V_{DD}\$ and \$V_{SS}\$. I'd aim for no more than 5.5 volts because some supplies may produce transient anomalies that are unexpected.

Can Vss be lesser than 0V, say -5V as in my case? Or should Vss be 0V?

Yes, it can be less than 0 volts providing that the difference between \$V_{DD}\$ and \$V_{SS}\$ is as per above.

Will this cause us any issues? Such as output offset, and drift?

When using an offset 0 volts it means that your input rails are naturally (in quiescent) close to one power rail or the other so, if you look at this figure in the data sheet you can see there won't be any notable increase in input offset voltage: -

And, if you are worried about differences in input bias and offset currents when the 0 volts is offset, then I don't see it as a problem: -

edited Mar 14 '23 at 15:35

answered Mar 14 '23 at 11:26

Andy aka

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Thank you very much for your answer. Could you please tell me when you say 50mV from each power rail, from where in the datasheet you got the value of 50mV? – Mar 14 '23 at 15:08
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@Newbie I rounded it up to be 50 mV. If you look at the top image in my answer and the blue box you can see that the output can typically get to within 35 mV of \$V_{SS}\$ and 45 mV of \$V_{DD}\$. – Andy aka Mar 14 '23 at 15:11
Also, Vdd or Vss, the difference should be in between 6.5V, right? Like, whether all op-amps are like this? Like, they can handle any potential between the two supplies as long as their Vdd-Vss is less than 6.5V, right? But in the Power Supply limits, it is mentioned as between 2.2V to 5.5V? What is this value then? Could you please clarify? – Mar 14 '23 at 15:13
@Newbie do you understand difference between absolute maximum and rated operating range of chips? – Justme Mar 14 '23 at 15:18
6.5 volts is the absolute maximum rating and, if you are thinking of going to that limit then don't because you are borderline with the device failing. In fact, this device should be used at voltages no greater than 5.5 volts (if you want to maintain the performance stated in the data sheet). Anything above 5.5 volts isn't really guaranteed @Newbie – Andy aka Mar 14 '23 at 15:18
@Justme, yes. But I am not getting clarity on this. – Mar 14 '23 at 15:22
Thank you @Andyaka. I'd just like to clarify whether what is the meaning of Power Supply voltage on this data sheet. Like when it is mentioned as a Power Supply voltage between 2.2V to 5.5V, what does it imply with respect to the Vdd-Vss? – Mar 14 '23 at 15:24
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@Newbie it means that \$V_{DD}\$ minus \$V_{SS}\$ should be between 2.2 volts and 5.5 volts for guaranteed performance. – Andy aka Mar 14 '23 at 15:25
Thank you @Andyaka – Mar 14 '23 at 15:27
Just a small clarification after going through the datasheet, if you see the second paragraph on section 3.3,page 17, it is mentioned, "Typically, these parts are used in a single (positive) supply configuration. In this case, VSS is connected to ground and VDD is connected to the supply. " So, it doesn't recommend negative supply rails? – Mar 15 '23 at 06:06
These parts can be used with a negative supply rail. It's just saying that it has a big usage in 5 volt supplied systems. – Andy aka Mar 15 '23 at 08:58
Thank you for the clarification. While trying to understand, I see that the output of the opamp cannot be greater than Vdd and lesser than Vss. If that's the case, could you please clarify on how the common mode input voltage ranges can be greater than Vdd by 0.3V and lesser than Vss by 0.2V? How does the opamp accept these inputs when the supply limits are exceeded? – Mar 15 '23 at 09:27
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This is sufficiently unrelated to the original question (and requires quite a long winded answer) that it would make sense to ask this as a new question @Newbie – Andy aka Mar 15 '23 at 09:40
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Sure, thank you. I will ask a new question to get this clarified. – Mar 15 '23 at 09:41

Simon Fitch · Answer 2 · 2023-03-14T11:59:31.663

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issues such as output offset, and drift?

There won't be any issues like that. The op-amp doesn't even know what zero volts is, so whether you powering it from ±2.5V, or -50V and -55V, that changes nothing.

Is it the difference between two voltages that must not be more than 6.5V?

Correct, as long as \$V_{DD} - V_{SS} \le 6.5V\$ you won't break anything, and as long as \$V_{DD} - V_{SS}\$ lies between 2.2V and 5.5V, the op-amp will perform as per specification.

Can Vss be les than 0V, say -5V as in my case? Or should Vss be 0V?

\$V_{SS}\$ can be anything you want, -1000V, or +1000000V as long as \$V_{DD} - V_{SS} \le 5.5V\$. It is not necessary for \$V_{SS}\$ to be 0V. Your value of \$V_{SS}=-5V\$ is fine. However, You must ensure that input potentials never fall outside the range \$V_{SS}-0.2...V_{DD}+0.3\$ (see page 4, "Common-mode Input Voltage Range"). Don't forget that your chosen point of "zero" volts is completely arbitrary anyway. You're the only one who cares what zero is, the op-amp (just like any component) only cares about the differences between voltages at its pins. You'd do well to give that some thought. When you "get it", it's one of the single greatest epiphanies a beginner can have.

The minimum supply voltage is 2.2V. Even if Vss is -4.3V, the device condition of Vdd-Vss=6.5V is satisfied right?

Yes, as long as \$V_{DD} - V_{SS}\$ lies between 2.2V and 5.5V, you're good. Even between 5.5V and 6.5V difference, your device won't necessarily work, but it won't be damaged (other conditions notwithstanding).

edited Mar 14 '23 at 11:59

answered Mar 14 '23 at 11:39

Simon Fitch

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Thank you very much for the answer. I'd just like to clarify whether what is the meaning of Power Supply voltage on this data sheet. Like when it is mentioned as a Power Supply voltage between 2.2V to 5.5V, what does it imply with respect to the Vdd-Vss? Also, what is the meaning of rail-to-rail opamp? – Mar 14 '23 at 15:21
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@Newbie "Power supply voltage" just means the difference \$V_{DD}-V_{SS}\$. It's up to you to decide the value of those two individual potentials, all the op-amp "sees" is the difference between them, its "power supply". – Simon Fitch Mar 14 '23 at 16:01
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@Newbie "Rail" refers to a power supply potential. \$V_{SS}\$ is the lower "rail", \$V_{DD}\$ is the upper rail. "Rail-to-rail" output is a claim that the output is able rise or fall in potential almost all the way to either extreme. Many op-amps can only get to within a volt or so of those extremes. Others, like the LM358 can get within a few millivolts of the lower rail, but only within 1.5V of the upper rail. The word "Rail" comes from the analogy of a pair of long rails, or bus-bars, carrying a potential difference to many devices connected across them. – Simon Fitch Mar 14 '23 at 16:06
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@Newbie By "almost all the way to a rail" I mean to within a few millivolts of it. I don't know of any device which guarantees an ability to actually achieve \$V_{OUT}=V_{DD}\$ or \$V_{OUT}=V_{SS}\$. Perhaps they exist. – Simon Fitch Mar 14 '23 at 16:23
Thank you very much for these clarifications. Much appreciate it! :) Thanks! – Mar 14 '23 at 17:49
Just a small clarification after going through the datasheet, if you see the second paragraph on section 3.3,page 17, it is mentioned, "Typically, these parts are used in a single (positive) supply configuration. In this case, VSS is connected to ground and VDD is connected to the supply. " So, it doesn't recommend negative supply rails? – Mar 15 '23 at 06:06
The word "typically" tells you everything you need to know. Many applications have only a single supply of 0V/5V available. That would be a situation ideal for this kind of device, and is a "typical" scenario. However, others may have rails of +1.5V ands -1.5V (a "supply" of 3V), from a couple of AA cells, less typical, but still absolutely fine. There are *not* saying "we don't recommend negative \$V_{SS}\$. – Simon Fitch Mar 15 '23 at 07:19
Thank you very much for the clarification @Simon Fitch – Mar 15 '23 at 08:38
While trying to understand, I see that the output of the opamp cannot be greater than Vdd and lesser than Vss. If that's the case, could you please clarify on how the common mode input voltage ranges can be greater than Vdd by 0.3V and lesser than Vss by 0.2V? How does the opamp accept these inputs when the supply limits are exceeded? – Mar 15 '23 at 09:28
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@Newbie Everything depends on the input circuitry. It may be possible (as the datasheet seems to imply) for input potentials to exceed the rails by a small amount, without compromising the op-amp's ability to "calculate" the difference between them. The term "common mode" means "when both inputs are at almost the same potential", so it's really saying that you can take *both* inputs just above \$V_{DD}\$, or both inputs just below \$V_{SS}\$, and the op-amp will still do what it's supposed to. – Simon Fitch Mar 15 '23 at 09:52
@Newbie What happens when one input goes outside this range upwards, and the other goes outside downwards, I have no idea. – Simon Fitch Mar 15 '23 at 09:55
Thank you for your answer. So, If my supplies are 0.5V and -5V, my inputs to the op-amp can be only between: (0.5V to 0.8V) or (-5V to -5.2V), right? – Mar 15 '23 at 10:00
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@Newbie With supply rails of -5.0V and +0.5V, don't let your inputs fall below -5.2V, or rise above +0.8V. Anything between those extremes is fine. – Simon Fitch Mar 15 '23 at 10:19
Thank you. So, one input can be -3V and the other input can be 0V, right? – Mar 15 '23 at 10:47
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@Newbie that is correct. With negative feedback, that situation is unlikely (you'll need to learn why elsewhere), but it's a perfectly acceptable pair of inputs, with supplies of +0.5V and -5.0V. – Simon Fitch Mar 15 '23 at 12:27

score 2 · Answer 3 · answered Mar 14 '23 at 11:27

Will this cause us any issues?

No.

All that matters is the difference between VDD and VSS pins. Because an op amp has no idea about the circuit ground. So, as long as the difference is kept within the limits you're fine (e.g. VDD to ground, VSS to -6.5V).

If a non-rail-to-rail op amp (e.g. VOL = VSS + 0.5V) is used in an application where the output of the op amp must swing to 0V (or slightly lower), then VSS can be, say, -1V while VDD is, say, 5V to make sure the output can swing to 0V or lower.

Or a similar thing can be seen in measurement devices such as DMMs. They are supplied with a single battery but they are supposed to measure negative voltages as well. Some op amps inside are fed with +3.3V and -2.5V, for VDD and VSS, respectively.

Thank you for your answer. Could you please let me know what is the difference between rail to rail and non-rail to rail? — , Mar 14 '23 at 15:31
@Newbie A rail-to-rail op amp's output can hit the rail levels i.e. quite close (i.e. within millivolts) to VDD and VSS. The others are not capable of doing this. For example, the famous LM358's output cannot be higher than VDD-1.5V. This means that you can't get 5V output when the supply is 5V. — Rohat Kılıç, Mar 14 '23 at 15:54

score 1 · Answer 4 · answered Mar 14 '23 at 15:30

So the supplies are very unbalanced.

How would the op-amp "know" that? It's only point of reference is the input common mode voltage range - i.e. the range of voltages expected on the (+) input, and you said nothing about that.

All that the op-amp cares about is that the input common mode range and output voltage range are within limits. These limits are relative to the supply rails, yes, thus we need to know all of the following to make any statements:

The rail voltages.
The input common mode voltage range from the datasheet, relative to supply rails.
The input common mode voltage range in the application you're using the op-amp in.
The output voltage range from the datasheet, relative to supply rails.
The output voltage range in your particular application.

score 0 · Answer 5 · answered Mar 14 '23 at 14:25

While I agree with all the good answers, I also think that a safety issue should be reminded.

Even if what's important is the VDD - VSS, for supply rating, having in the circuit some reference voltage very far from them can become an issue, at least on transients.

Just suppose that at steady-state VDD - VSS = 5V, but a power/signal/earth ground is VSS - GND = 100V. If VDD and VSS are decoupled to GND with a pair of capacitors, that can pose a serious issue during any event that charges/discharges them.

Opamp with unbalanced rails

5 Answers5