# Significant error with unity-gain feedback op-amp

I used this OP97 op-amp to make a simple unity-gain feedback circuit. I would expect the output in this case to follow the input with very little error, however the output is actually measured at $$\ 2.7 \text{ V}\$$ rather than $$\ \approx 3.1 \text{ V}\$$ as expected.

This seems like a fairly large error for this buffer. I checked the data sheet and it appears that I'm operating the device well-within the allowable supply range, so I'm not sure what the error may be.

simulate this circuit – Schematic created using CircuitLab

• Can you show your PCB design? This op amp has very high input impedance and is susceptible to very small leakage currents which may cause increased offset. Check the datasheet section on guarding. – ScienceGeyser Feb 23 at 0:25
• This is actually a breadboard design, so the schematic is pretty much all there is. I'm powering it from the ADALM2000. – HallEffect Feb 23 at 0:29
• Check to see if the offset exists with other input voltages. Or put a sine wave on the input and look at the distortion as the input amplitude approaches the rails. – ScienceGeyser Feb 23 at 0:41
• It seems ironic you chose an Op Amp with Offset voltage =20 μV maximum and then realized 400 mV of offset as good as a uA741 – Tony Stewart Sunnyskyguy EE75 Feb 23 at 1:14

Figure 1. From the datasheet.

Note that when running on a ±15 V supply that the inputs can be taken to within 1 or 1.5 V of the supply rails.

If that deviation remains constant at ±4 V then you could expect the inputs to work in the range ±2.5 V to ±3 V. I can't see where you could work that out from the data sheet.

• Thanks a lot for the quick response. So then in that same table in the row right below the one you pointed out, for "output voltage swing" it says +/- 13 V. I guess since this is a unity gain buffer, if I were operating at +/- 15, I should expect the more limiting constraint to apply? – HallEffect Feb 23 at 0:18
• I'm afraid so. Have a look at rail to fail op-amps but be aware that they don't have strong output "pull" when close to the results. – Transistor Feb 23 at 0:34
• @Transistor is "rail to fail" a typo or are you mocking them? (the correct term is "rail to rail") – user253751 Feb 23 at 9:21
• @user253751. Ha! Typo, thanks. It should be "rail-to-rail". – Transistor Feb 23 at 10:15
• yes you want rail-to-rail. The original is a rail-to-fail! – Brian Drummond Feb 23 at 13:44

The problem is the input voltage range, you may want to switch to a rail to rail opamp (some here). A rail to rail opamp has no resrictions on the input vs the rail voltage. There are also rail to rail input and rail to rail output opamps, make sure that at minimum its rail to rail input.