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# Opamp with unbalanced rails

#### FreshmanNewbie

##### Full Member level 4
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?

#### ELECTRONICIENSENIOR

##### Member level 2
Hello,

The most important is :

High DC Precision:
-VOS Drift: ±20 nV/°C (maximum, VDD =5.5V
Easy to Use:
- Rail-to-rail input/output

And on page 3 :

Electrical Characteristics:Unless otherwise indicated, TA= +25°C, VDD= +2.2V to +5.5V, VSS= GND,

In your case the op will be supplied with 5,5 V less than the max. In the Typical Application Circui page 2 the polarisation is called Vcc/2.It is quite the same symetrical application.
In the past when input nias current was not very low (LM101 for ex) a virtual ground was made with an other op-amp, here is a good article :

In your case, two equal resistors, for ex 10K, with decoupling, but if consumption is critical, two higher values, 1M, but with another op amp mounted as follower (MCP6V82 has two)

#### KlausST

##### Super Moderator
Staff member
Hi,

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?

if you say it is supplied by +0.5V and -5.0V then indeed it means:
* +0.5V w.r.t. GND
* -5.0V w.r.t. GND

But I guess your OPAMP does not have any GND pin. If so it will never know that you use "unsymmetrical" power supplies.
The problem for the OPAMP even does not exist. It just sees the voltage between the supply pins and IN and OUT voltage w.r.t. it´s supply pins.

And for sure they all need to meet the datasheet specifications.These voltage specifications usually refer to V+ or V- (the supply pins) but never to GND (unless the OPAMP has a dedicated GND pin)

****
Your given datasheet never uses "GND" as reference. All searches for "GND" find only "VSS = GND".
This just tells you the test condition.

Klaus