# Unity Gain Op-amp - 0 volts input giving 1.x volts output. Why?

I have this setup going into an MCU. I'm using a multimeter to verify voltages, and I notice whenever i set the input voltage to 0, the output of the OP amp is over 1 volts. When i increase voltage it starts following it. THe minimum volts it is giving is the 1.x volts. Why is this so? Opamps have a limit in the voltage they can output. According to the datasheet, the output voltage usually ranges from -13.5 to +13.5 volts given a +15 and -15 volt supply. This means that the opamp output cannot go below the negative supply plus 1.5 volts and cannot go over the positive supply minus 1.5 volts. In your case, the output cannot go under 1.5 volts and over 3.5 volts.

A simple solution would be to provide a negative voltage supply. This can be done with a simple circuit like this from this page. You supply a frequency into C1 and a negative voltage will be present at the negative plate of C2.

Alternatively, you can substitute your current one with a "rail-to-rail" opamp which can get much closer to the supply rails.

• Thanks for the 'feedback' :) What would you do differently if you wanted to read a 0-4.9v sensor without loading it ? I figured an Op-amp would work but i'm sure there might be better circuits. – DonP Apr 28 '18 at 6:50
• @DonP I would just feed the sensor output directly into the micro controller which shouldn't load the sensor it too much. – Alex Apr 29 '18 at 3:42
• @DonP if the input sink current of controller is within the supplying current limit of sensor, I would connect it directly. elsewise, I would take Rail to Rail OpAmp. e.g. OPA836 – HerrderElektronik May 2 '18 at 7:30

You are lucky to be seeing any behaviour similar to an op-amp at all at that rail voltage. It really prefers to run at +/- 15v, this is an old-skool op-amp.

If you look at the OP27 data sheet, you will see the 'input voltage range' specification is typically +/- 12.3v with a +/- 15v supply, which is 2.7v away from either rail. So the typical op27 doesn't even have a valid input range when supplied with less than 5.4v. The guaranteed specification is +/- 11v, so no valid input range for less than an 8v supply. If you look at figure 12 on page 9, they don't bother to plot a supply voltage of less than 7v.

The output voltage swing is also limited, typically +/- 13.5v with a light load, so 1.5v away from either rail, which is perfectly adequate when you have the headroom of 15v supplies.

Have a look at the functional block diagram on the first page of the data sheet to see the reasons that the input and output range get nowhere near to the rails.

If you want to have outputs or inputs close to a rail, you need to choose a rail to rail (R2R) opamp. LM324 is not true R2R, but it will run on 5v and get you to the -ve rail, however it still won't get within 2v of the positive rail.

• @RohatKılıç I don't think so because the gain should be 1 since the output is connected to the inverting input. – Alex Apr 28 '18 at 5:36
• @AHL Ah yes, this was a buffer. I couldn't see the picture because imgur is "cursed" here in Turkey. Grrr. Deleting the comment. Thanks. – Rohat Kılıç Apr 28 '18 at 5:45