Advice for low voltage logger OP AMP

I need to measure (and log) voltages lower than 1V.
I have a good program for logging the voltages and it uses the internal 1.1 V reference.
The max voltage I'm trying to measure is 1.35 volts.
However, most voltages will fall below that (voltages are a AA battery across a leaf)
Unfortunately, I have just learned the hard lesson that my UNO can't easily measure less than 1 V
My logger files have been filled with zeros
I have troubleshot all sensors and circuits and I find logger writes to SD card when enough voltage is present, suggesting the problem is sensitivity

Looks like I'm going to need to reconfigure my prototyping on the logger shield to incorporate an Operational Amplifier
I have 4 sensors and I was wondering if the TLV2264ID would be an appropriate Op Amp? http://www.mouser.com/ProductDetail/Texas-Instruments/TLV2264ID/?qs=afYny40WCj1UBuYRmyjypg==

Would using a tiny voltage for AREF be an alternative?

Check out the LT1215. It’s available in a DIP package and is designed for use with 5V systems.
Your statement suggest that there may be more going on than you are aware or have let on. One volt should yield an analogRead value of 200 , not 0 , so unless you can explain why this is so there is no reason to think that using a fancy op amp will solve your problem. Garbage in => garbage out. If you had 1 volt across the leaf even the cheapest 741 op amp could read that. I suspect the problem is more basic. The fact that you have not posted any schematic or voltages measured or a diagram of your test setup suggests that perhaps maybe you don’t have a lot of experience with electronics. If this is not so, please post schematic or photo of one of how you are collecting the data. Also post your code. Post a photo of the actual setup and post a photo of a digital multimeter showing the voltage across the leaf. Give wire lengths from UUT to arduino. Don’t forget to post the code.

LT1215.pdf (339 KB)

These are inexpensive and readily found.
http://www.ti.com/lit/ds/symlink/lm3900-n.pdf

Your Arduino is sitting on the leaf, this may give you some problems.

I am not sure if I am understanding you correctly. Please post a schematic (photo of a hand drawn schematic) and explain what you just said because I am not following the significance of the jumper or the aligator clip.. I need to see a proper schematic that shows how the battery and resistor are connected and what the jumper is. Why are you using a jumper ? You know that is not going to make a good connection. You need to use an aligator clip and two pieces of aluminum foil that make a sandwich with the leaf in between and the aligator clip pressing the foil against the leaf on both sides. Even without the schematic, it appears to me that the problem is not so much electronics or voltages too low as it is poor test setup. The arduino should be reading the same thing as a multimeter (which I still DON'T see in the photo and which was one of my requests) We need to have some controls here. There is absolutely no point in using a uC to measure a voltage you cannot read on a meter. Until you have a meter next to the leaf ALL the time displaying the SAME voltage that is going to the arduino, there is no point in continuing this experiment.
I want to see the following:
1- Meter connected to analog input displaying the voltage that is goin to the arduino.
2- A schematic of the whole setup.
3- explanation for the resistor and battery
4-better physical connection. Lose the jumper and replace it with the foil sandwich I described.
5. serial monitor terminal capture using a serial terminal capture program like Clear Terminial that allows you to save the serial buffer to a text file which you can post.

That 100R resistor is way way too low. The resistor needs to bein the same order of magnitude as your leaf. Measure your leaf with a resistance meter and choose the same sort of value for the pull down resistor.

There is a very good reason why Mike said what he said. Do you understand what it is ?
Replace the leaf with a resistor symbol and reread what Mike said.
If you do what he says, what will be value read by the arduino ?

What would happen if you swap the positions of the leaf and the resistor in the schematic ?

Would using a tiny voltage for AREF be an alternative?

No. There is a limit on how low you can take Vref, it is about 1V.

I have a good program for logging the voltages and it uses the internal 1.1 V reference.
The max voltage I'm trying to measure is 1.35 volts.

You can not measure any voltage greater than the reference voltage, so if you need to measure 1.35V then the reference voltage has to be at least 1.35V.

If take away the resistor to ground .....

Then you have a simple resistor to your battery and you will never see any variations unless the leaf is has so high a resistance you have in effect a floating input and the readings while changing are meaningless because they only change because of interference pickup.

While it is clear you want to measure the resistance of a leaf I wonder why. Is this a one off measurement or do you want to monitor it over time as it drys out. Either way I am not sure where it gets you.

It sounds like you need an AREF of 1.5V.

You started this post asking if you should use an Op Amp.
Do you KNOW HOW to use an Op Amp ?
I would suggest a Voltage Follower input => Non- Inverting Amp ( A=3)=>

You are using you Arduino as an ohmmeter since the input voltage is proportional to the resistance of the leaf. Why not use three batteries in series for a higher supply voltage which will increase the a/d resolution rather than resorting to using an op amp to achieve the same thing?

Replace your measuring probe with two pieces of aluminaum foil paper clipped to the leaf with an aligator clip on the paper clip.
You can adjust the shape of the paper clip to adjust the spring tension on the leaf.
Once you have good connections at each end of the leaf, use those to measure the resistance of the leaf.
Post the resistance of the leaf.
Replace the 100 ohm resistor with one close to the same resistance as the leaf.
Measure the Voltage on the leaf with a meter after changing the resistor.
Take some analog readings with the new setup and post the values.

Your statement suggest that there may be more going on than you are aware or have let on.

Mouth-parts of a LIVE insect ? cage ? screen ?

It seems my original suspicions have been confirmed. There is indeed a lot more going on here that you have let on.
I have no experience using live insect parts as resistive components in a circuit. Now I really don't know what to suggest.
What started out as a leaf and a battery is now Science Project in a Tree.
Go figure...
Somebody help me ...
Can you a post a datasheet for the insect ? XD

Use the aa battery itself as a reference. This should make it very easy to calculate the leafs resistance…

I can't increase the voltage. The probe that touches the leaf to complete the circuit in my test circuit is actually the mouthparts of a live insect (stink bug). I can't go higher, or I will be affecting the feeding behavior. The bug is contained on the leaf in a small cage. At the base of the cage just above the leaf surface there is a copper screen, and that is where the sensor end of the circuit connects.
By standing on the screen and inserting its mouthparts into the leaf, or just touching the leaf, the circuit is completed

Use the aa battery itself as a reference. This should make it very easy to calculate the leafs resistance..

Really ?

The probe that touches the leaf to complete the circuit in my test circuit is actually the mouthparts of a live insect (stink bug). I can't go higher, or I will be affecting the feeding behavior.

Are you quite sure that 1.5V is not affecting the feeding behavior I would have thought it would have.

I have proven the concept in the field already using a commercial voltage logger. But, the result showed ghosting between channels and also a lot of noise. I want better control.

So you want something better than the commercial system. That calls out for using an op amp, why you are trying to do without it.