# Opamp with unbalanced rails

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!!! Mar 14 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? Mar 14 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. Mar 14 at 12:40

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: -

• 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 at 15:08
• @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 $V_{DD}$. Mar 14 at 15:11
• @Newbie it means that $V_{DD}$ minus $V_{SS}$ should be between 2.2 volts and 5.5 volts for guaranteed performance. Mar 14 at 15:25
• 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 Mar 15 at 9:40
• Sure, thank you. I will ask a new question to get this clarified. Mar 15 at 9:41

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).

• @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". Mar 14 at 16:01
• @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. Mar 14 at 16:06
• @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. Mar 14 at 16:23
• @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. Mar 15 at 9:52
• @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. Mar 15 at 10:19

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 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. Mar 14 at 15:54

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:

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

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.