LM4040 AREF Pin Voltage Drop

Hello,

i'm having a little trouble getting my external reference to work properly.
I'm using Arduino Nano and the LM4040 5V and connected it through a 1k resistor to Vcc. It's basically just this configuration, only with the 5V instead of 4.096V and 1k resistor instead of 904.

When measuring the REF Voltage (not connected) with my voltage meter, i get 4.98V, which is fine.
But as soon as i connect it to the REF Pin, the voltage drops to 4.84V.

I have set the analogReference(EXTERNAL) in my code.
I've already tried different resistors, but the voltage drop is always there.

Does anyone know what the problem might be?
It's messing up all my analogReads.

Thanks and have a good day.
-Maik

Hi @ma_wu,
The lowest current for the LM4040 -5V to work is:
IRMIN Minimum Operating Current 54 uA.
Ref: https://www.ti.com/lit/ds/symlink/lm4040-n.pdf?ts=1634661159426&ref_url=https%3A%2F%2Fwww.google.com%2F page: 25

If you have it for 5V and you are feeding it with 5V, theoretically the current will be 0 uA and the LM4040 -5V is not working.
As it is not working, any measurement of values becomes invalid.
Try feeding it with some voltage like 5.3V or 5.4V.

RV mineirin

I do not know very much about the Vref input except that it is not high impedance, so it will draw some current from an external source.

I can think of couple of possibilities - There is an internal resistor -

Or, you might be "too close" to Vcc. I'm pretty sure the external reference has to be less than Vcc.

Which one.
Many Arduinos with different processors and logic voltages are confusingly called "Nano".

If you mean a classic 5volt/16Mhz Nano, then show us your code/diagram, how you power it, and a picture of the setup. Also tell us why you use an external Aref.

Don't forget that the Aref in has an internal resistance of 32k, which makes a voltage divider with the reference supply resistor.
Leo..

You can't run a 5.0V reference from 5V+/-5% !!!!
You need definite headroom.
Also you can't risk having VREF > Vcc, that's never going to work. With 5V logic a 4.096V reference is the obvious choice.

Or 4.5V, like the REF5045

If... OP wants to measure absolute voltages in that range, then an ADS1115 (with internal 1.xxvolt reference and 4.096 volt PGA) could be a much better choice.
Let's wait until we know what needs to be measured.

Headroom for the LM4040 could be as low as 4.65 (Nano on USB supply) - 4.096 = 0.55volt.
Leo..

Thanks for the fast answers!

Feeding it with 5V might be the problem.
I need the reference to make the measurement of my battery voltage more precise and stable while using this voltage sensor:

But instead of getting a new LM4040 with a 4.096V output, I can just work with the 4,84V I'm getting, right?
This value should be stable and therefore should work.
Or are there any other risks I'm not considering?

Thank you and have a good day.

No need to have an external reference for that.
The Nano has a stable internal voltage reference of about 1.1volt, which you can select with code.
All you do is divide battery voltage down to about 1volt with a two-resistor voltage divider,
and add this line to setup().
analogReference(INTERNAL);
More info here.
Tell us which battery if you need help with resistor values.
Leo..