Op amp not working as expected

Thread Starter

roci75

Joined Feb 26, 2024
6
Hello guys, I bought an op amp from mouser, the LMP7721 (data sheet below).

I have been testing it, I soldered it to a break out board.

after connecting 3.3v supply from an arduino uno to v+ and v- and connecting in- to vout I measured between ground and in+ 1.2 to 1.8 V.

if I disconnect vout from in- , vout shows 0v
And both in- and in+ show 1.2 to 1.8 V.

can I assume that the op amp is broken?

ps ( I actually bought 2 LMP7721 but I’m waiting on desoldering alloy to be able to switch the other one with the one is in the break out board).

Any way, I hope some one can help me, I am grateful for any assistance.

link to data sheet:
https://www.ti.com/lit/ds/symlink/lmp7721.pdf
 

Papabravo

Joined Feb 24, 2006
21,225
Hello guys, I bought an op amp from mouser, the LMP7721 (data sheet below).

I have been testing it, I soldered it to a break out board.

after connecting 3.3v supply from an arduino uno to v+ and v- and connecting in- to vout I measured between ground and in+ 1.2 to 1.8 V.

if I disconnect vout from in- , vout shows 0v
And both in- and in+ show 1.2 to 1.8 V.

can I assume that the op amp is broken?

ps ( I actually bought 2 LMP7721 but I’m waiting on desoldering alloy to be able to switch the other one with the one is in the break out board).

Any way, I hope some one can help me, I am grateful for any assistance.

link to data sheet:
https://www.ti.com/lit/ds/symlink/lmp7721.pdf
The language of electronics is the schematic – not English.
 

Thread Starter

roci75

Joined Feb 26, 2024
6
No. You cannot measure input voltages directly because the inputs have very high input impedances.

Show us a diagram of your test circuit.

Here are two simple test circuits:

View attachment 316361
Hi MrChips, just wanted to thank you for your help, I tested the op amp with the non inverted circuit you showed ant it is working now.
I assumed that when there is no input to the op amp it would out put 0v but apparently it doesn’t work predictively when there is no input.
 

MrChips

Joined Oct 2, 2009
30,795
It is a newcomer mistake to think that when there is no input it is at 0V.
This is not the case for high impedance inputs on analog and digital circuits.
A high input impedance means that the input is an antenna that can pick up every radio, TV, cell phone, bluetooth and 50Hz/60Hz AC line voltage signal.
 

BobTPH

Joined Jun 5, 2013
8,943
after connecting 3.3v supply from an arduino uno to v+ and v- and connecting in- to vout I measured between ground and in+ 1.2 to 1.8 V.
Why are you measuring the voltage at a floating input? That is meaningless. Connect the input to a known voltage and measure the output.
 
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