Maker Pro
Maker Pro

Op-Amp voltage follower output different from input

jgauthier

Mar 22, 2013
63
Joined
Mar 22, 2013
Messages
63
I am using this op-amp. I have a positive rail (about 10V) and a negative rail (about -13V) explained it in this other post.
The op-amp is basically being used a voltage buffer.

In my application I have 10 ADC inputs reading current transducers. I am offsetting the current transducer by 1.65V (half of my ADC voltage reference). I felt like an op-amp would be a good use-case for this. the 1.65V comes from a voltage divider after a 3.3V regulator.
Since there isn't any load on the 1.65V, I felt like a voltage divider should be okay. I used the op-amp to remove the impedance from the voltage divider.


Now, that said, the op-amp input is 1.65V, but the output is 1.8V. I am currently unsure why this is. Could it be the imbalance on the +/- rail? I am also using the Op-amp as a buffer for 1.65VAC signal, and the signal is equal. Maybe there is something else after the op-amp that is somehow adding 200mV. I need to explore the circuit more. I wanted to put this out here for any other tips I might have missed. Thanks!

1683481651540.png
 

jgauthier

Mar 22, 2013
63
Joined
Mar 22, 2013
Messages
63
What is the output with 0V in?
Hm. Well, I removed R22 and R23, for the voltage divider input. I verified on the schematic that was not used anywhere else.
But, the input showed 1.7V and the output now showed 3.9V.

Pictured here, pins 1,2,3 are the ones I brought up in the initial post.
Pin 5 is 1.65VAC that is obtained through the voltage divider shown in the original post with R2 and R12.
Pin 10,9 are used as a summing input for 1.65VAC offset with with buffered 1.65VDC.
Pin 8 is the output of that in ADC range (0-3.3V)

I removed the op amp, and as I suspected the place where pin 3 was in 0V.
So, something else is wrong causing the op-amp to have a voltage on pin 3, but since there are no other connections it must be happening inside the op amp?


1683484482777.png
 

Alec_t

Jul 7, 2015
3,590
Joined
Jul 7, 2015
Messages
3,590
Leaving input pins 12 and 13 floating is not a good idea. It could lead to unpredictable effects, such as oscillation.
 

jgauthier

Mar 22, 2013
63
Joined
Mar 22, 2013
Messages
63
I removed all the other inputs, and put the 1.65V back into pin 3. 1.8V comes out. Still.
I checked the datasheet and it agrees with you:
"Any unused channel of the ADA4522-1/ADA4522-2/ADA4522-4 must be configured in unity gain with the input common-mode voltage tied to the midpoint of the power supplies"

I need to see how I can potentially run a jumper(s) for that.
 

jgauthier

Mar 22, 2013
63
Joined
Mar 22, 2013
Messages
63
Unfortunately, utilizing the last channel did not change the result. I connected my 1.65V input to the unused channel, and the output is also 1.8V. I can however, easily input 0V to that channel to test it. But I have to put it down for now. Thanks for the help.
 

bertus

Moderator
Nov 8, 2019
3,321
Joined
Nov 8, 2019
Messages
3,321
Hello,

Have a look at this page:
Or this page:

Bertus
 
Top