I can use the external Vref with my ADC now.
I noticed something that is confusing.

Vref+ is at 5 V & Vref- is GND, derived from a TL431 Regulator.

I am varying the ADC with a multiturn pot for experimenting, Pot uses Vdd..

I noticed that the ADC is jumping too much, just not a few digits. Even with the slow refresh I am using.
So I changed the Vref to internal VDD and it is stable. Stable enough for me although there may be a little changing but it is not noticeable due to slow refresh rate.

Is it because that the TL431 reference has noise?
The Vdd is a standard 7805. I believe it might have more noise.

And is the internal Vref is stable ( showing to be stable ) because the ADC noise and Vref+ noise is same coming from the 7805 and they cancel out each other ?

 

Yes, that canceling-out process should certainly happen.What you're really measuring is a ratio between Vref and the voltage on the analog input, and if they're already a ratio, as you'd get by reading a pot, noise should cancel.

 

Do you have good decoupling on both VDD and the TL431?
And short connections (0V and 5V) between the TL431 and the ADC?

 

Do you have good decoupling on both VDD and the TL431?
And short connections (0V and 5V) between the TL431 and the ADC?

Vdd is decoupled with 100nf plus 7805 has decoupling.
TL431 input output is decoupled with 100nf. In short everything is decoupled with it's own monolithic 100nf ceramics.
The distance between Vref and TL431 is like 2 inches but connected with fly wire for testing.

So with external Vref, is the noise dominating from Vdd supply (coupled into POT to ADC ) or the TL431 Regulator ?

 

So with external Vref, is the noise dominating from Vdd supply (coupled into POT to ADC ) or the TL431 Regulator ?

Either. As John P said, you are measuring the ratio so if either varies, the ratio and hence the reading will vary.

 

Why there is no variation using Vref as internal Vdd ?

 

I'm asking because I don't know; is this a case where it might be worth while to use a scope to detect any frequencies on the power lines, then use that frequency to attempt to calculate a suitable capacitor for decoupling? As opposed to just taking a guess?

 

I do have a scope but I dunno what I am looking for.
I can always use the Vdd as Vref but this project will be duplicated..so different circuits will have different Vdd as 7805 does not have strict 5.00V.
So I thought to cut the price I will use a TL431 as a reference. I can use a precision 5V reference from analog devices though but it would sky rocket the price I believe, as the PCB quantity will be like 50 to 100 units..

Will the ADC measurement differ from unit to unit if I use Vdd as reference, considering the fact that the LM7805 will have a varying degree of voltage say from 4.8V to 5.03V ?
The testing unit Vdd is at 5.03V.

The project does not require to measure the ADC's full scale but rather 2/3 of the ADC value will be used.

 

Why there is no variation using Vref as internal Vdd ?

Then the voltage which you are measuring will have the same variations as the reference, so the ratio is unchanged.

 

Hmmm. !

I will poke around some more

 

What do ya know..! ..It was the stupid reference after all.
Darn thing is Just too Noisy.

I guess there is no other way than to use a precision reference. Eh!

 

Just filter the heck out of the Tl431 reference voltage.
Use a series resistor to the A/D Vref input (maximum value depends upon the Vref input impedance) and then a large (100uF or more) to ground.

 

How much current are you putting through the TL431? 5mA appears to be the recommended value.
What supply voltage are you using to supply the TL431?
How can you use 5V supply on a 5V reference or am I missing something?

 

Hey @MrChips
Input is 12V. TL431 used as base drive for a 700mA transistor. am using the diagram shown in the datasheet. Simple circuit actually. Just a series regulator. With Input Output decoupling.
@crutschow
I can give that a try. Do you think it could work.

 

Why you using it as a series regulator with a transistor?
You just need to use the basic TL431 shunt regulator circuit with resistors (no transistor), and it should be used only for the reference voltage.
The rest of the circuit should be powered from a separate supply.

It would be good if you posted a schematic of what you are doing so we are all on the same page, because apparently we aren't, up to this point.

 

I will change it to a shunt will see how it does.

All the circuits are powered separately. The TL431 is used only as Vref+

 

@crutschow Guess what !
I used the RC filter like you said between Regulator and PIC Vref+ and the ADC is rock solid.
I am using the series 5V regulator from the data sheet.
I am using a 680R resistor between TL431 and PIC, plus a 100uf and a 100nf at the PIC Vref+ side.

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