I don't have them both on at the same time, only one is connected at a time.

It's supposed to be a non-inverting op-amp with a gain of 31. I'm testing with an input of 0 - 110mv. The LM358 seems to work just fine albeit the offset voltage makes it inaccurate at this range. The LT1097 does not work at all, it just puts out 0.55v for the whole input range. What gives? How do I get this to work?

more info: the output voltage is 0.55v whether the supply is 5v or 3v3. the output starts to change at 230mv non-linearly. at 286mv the output is 652mv, at 289mv the output is 702mv, at 293mv the output is 4.08v

i tried making a virtual ground, replacing R1 gnd connection with 2.5v but the result was basically the same, except the output doesn't change at all until the input is 3.3v, at which the output moves to 4v

i don't understand what's going wrong.

 

I suggest visiting the Linear Technology website and searching the application literature for that LT 1097 device. I am not familiar with it, but application notes are a very good source of information about specific products.

 

Why did You pick the LT1097 Op-Amp ?
.
.
.

 

The LM358 common mode input range includes the negative supply, the LT1097 doesn't so the
input is outside it's operational range.

https://www.analog.com/media/en/technical-documentation/data-sheets/1097fas.pdf

 

Why did You pick the LT1097 Op-Amp ?

I want to amplify signals that will be millivolts or less, and i would prefer something with more precision/accuracy than the LM358. The input offset voltage is too high for my application and it doesn't seem consistent enough to null in software. Anyway, I have about 10 of them that I bought a while ago so I'd like to figure out how to use them for my purposes.

The LM358 common mode input range includes the negative supply, the LT1097 doesn't so the
input is outside it's operational range.

How did you come to this conclusion? I'm not seeing any relevant parameters in the LT1097 datasheet. Not doubting you, I'd like to be able to find this out myself without experiments though

Anyway i gave it a makeshift split voltage (i think i messed up the first time i tried) and i got it working. unfortunately it is very imprecise, maybe because of the shoddy power supply situation. the rails don't look bad on a scope though, it's just the output that is all wacky. it gets higher/lower by perhaps 10% if i just bring my finger close to the op amp /etc.

 

I want to amplify signals that will be millivolts or less

LT1097 is inappropriate for that application.

How did you come to this conclusion? I'm not seeing any relevant parameters in the LT1097 datasheet. Not doubting you, I'd like to be able to find this out myself without experiments though

Staring you in the face if you know what to look for.



With a single 5V supply, the guaranteed input voltage range is 1.5-3.5V.

Also note that the output voltage range of the LM358 can be as low as 5-20mV. LT1097 output voltage swing can't get closer than 2V from the supplies (2-3V in your case).

The symbols you used in your "schematic" are terrible at conveying circuit intent. I don't usually give schematics drawn like that much consideration.

 

LT1097 is inappropriate for that application.

what makes it inappropriate? i was under the impression that a precision op amp is what i need. i now understand that i should be using a split supply and not expecting rail-to-rail outputs or inputs, but it seems like i could meet the necessary operating conditions. i just don't have a lot of experience with OPAmps.

Staring you in the face if you know what to look for.
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With a single 5V supply, the guaranteed input voltage range is 1.5-3.5V.

okay i think i see. i tried it with a +5/-5 as well, which gave outputs that were actually changing but highly inaccurate. if i'm understanding correctly, a +5/-5 supply should guarantee input voltage range of -3.5v - 3.5v and an input of millivolts should then be okay. however what i saw was a highly sensitive/noisy output voltage (fluctuating ±10% based on position of my hand or the oscilloscope or whatever) and an inconsistent gain/error.

The symbols you used in your "schematic" are terrible at conveying circuit intent. I don't usually give schematics drawn like that much consideration.

good to know, i will keep that in mind next time. thanks for taking a look anyway.

 

what i saw was a highly sensitive/noisy output voltage (fluctuating ±10% based on position of my hand or the oscilloscope or whatever) and an inconsistent gain/error.

Then it sounds like you have some high-impedance, sensitive nodes in your circuit that a picking up you hand movements.
That has nothing to do with the op amp, which is just amplifying whatever the node is seeing.
You need to build the circuit with connections as short as possible, and possibly putting it in a shielded box.
Long wires on a plug-in breadboard are a big no-no here.

 

If You would accurately outline what your project is supposed to do,
there would be a lot less guessing and speculation going on in this Thread.

My guess .........
It sounds like You need an Instrumentation-Amplifier, ( IA ).

Op-Amps need to be chosen based on Specifications,
not chosen by how many of them You have in your Junk-Drawer,
.
.
.

 

what makes it inappropriate?

It was inappropriate for the input signal level and power supply you chose.

 

I don't have them both on at the same time, only one is connected at a time.

It's supposed to be a non-inverting op-amp with a gain of 31. I'm testing with an input of 0 - 110mv. The LM358 seems to work just fine albeit the offset voltage makes it inaccurate at this range. The LT1097 does not work at all, it just puts out 0.55v for the whole input range. What gives? How do I get this to work?

more info: the output voltage is 0.55v whether the supply is 5v or 3v3. the output starts to change at 230mv non-linearly. at 286mv the output is 652mv, at 289mv the output is 702mv, at 293mv the output is 4.08v

i tried making a virtual ground, replacing R1 gnd connection with 2.5v but the result was basically the same, except the output doesn't change at all until the input is 3.3v, at which the output moves to 4v

i don't understand what's going wrong.

Neither one of these op amps are very good for what you are using it for, but the Lt 1097 needs a split power supply as it needs at least -1V on the -V terminal for its internal CCS. I would just use a battery.

The one you should be building with is a LT1006 or an OPA830

 

If You would accurately outline what your project is supposed to do,
there would be a lot less guessing and speculation going on in this Thread.

well in my defense, i expected to be able to use an OP amp for this in either a noninverting or differential configuration, so a substantial part of my being here is just trying to correct my understanding

Op-Amps need to be chosen based on Specifications,
not chosen by how many of them You have in your Junk-Drawer,

Hey that's the whole reason I chose the LT1097! it specifies a very low offset and I thought that i was meeting all of the operating conditions. i was just wrong about that but it seems like i could still meet them, if the supply rail/common mode voltage was the real showstopper

My guess .........
It sounds like You need an Instrumentation-Amplifier, ( IA ).

Yes, that is exactly what I need thank you. Since I have already corrected the supply rail issue I'm gonna try to add the buffers to replicate an instrumentation amp circuit, i'll see if there's any improvement for either the lt1097 or lm358 as a learning exercise. and for the matter of actually using an instrumentation amplifier in my project, i have bought an AD620 for that and i appreciate your advice.

 

A major purpose of these forums is to improve people's understandings. So this is the place to be, and to learn.