ADC in a micro - actual use of -Vref

[atferrari]

Thinking of ADC in a PIC 18F micro.

I need to perform AD conversions of a single ended signal ranging between 2900 and 3700 mV. By design I know that it cannot escape from there.

To increase resolution I should bring those values down to the 0 - 800 mV range and reduce Vref as much as the micro allows (and as long it is equal or higher than 800 mV), always with -Vref to ground. For the moment, I cannot change the level of the input range, so I am left playing only with Vrefs.

Today, reading the manual of a micro, I realized the possibility of deciding on the +Vref and -Vref settings. I never had clear what is the use of -Vref , thus my questions:

a) For my range 2900-3700 mV, would it make sense to set: +Vref =5,0V and -Vref =4,2V?

b) If the above is just stupid, could anyone at least give a concrete example of what's the use of -Vref? Kind of a grey area to me.

Gracias for any reply.

 

[ronsimpson]

With out looking at the data sheet for the PIC18F......
You could set +Vref at4.7V and -Vref at 2.9V. (in 8 bit mode) ADC of 0 at 2.9V and ADC of 255 at 4.7V.

If you set +Vref at 5V and -Vref at 2.5V you would double the resolution.
>>A LM431 can make a good 2.5V.

 

[large_ghostman]

which pic 18? and is 8 bit for speed?

 

[ronsimpson]

and is 8 bit for speed?

In this case he probably wants the highest resolution possible and speed is not as important.
Because I don't know the resolution I used "8 bit" as an example. I was trying to point out the DAC will measure form (-) to (+).

 

[large_ghostman]

In this case he probably wants the highest resolution possible and speed is not as important.
Because I don't know the resolution I used "8 bit" as an example. I was trying to point out the DAC will measure form (-) to (+).

Got you, sorry it confused me a bit. I asked the part number because I cant remember which of the 18f's I used had a limit on the difference between -REF and VDD, it probably wont apply but was going to check.

 

[Mike - K8LH]

Can you set -vRef at 2900-mV and +vRef at 3923-mV to provide 1-mV resolution on the ADC result (voltage = ADC result 0..1023 + 2900)?

 

[granddad]

I'm no guru on ADC ( mainly PIC24) but had acceptable results using 12 bit resolution ie 4096 steps and internal reference voltage of 4.096v or 2.048v as +ref -ref Vss whatever is available or external of same v . I get stuck on ADC timings and have to 'experiment'.

 

[large_ghostman]

Can you set -vRef at 2900-mV and +vRef at 3923-mV to provide 1-mV resolution on the ADC result (voltage = ADC result 0..1023 + 2900)?

Hi Mike, I think everyone else is trying to avoid that suggestion lol, the next question would be....how do I get an accurate reference for 3923 mV . More seriously though how far over is the wiggle room? Does it NEVER go above or below those two points? I am guessing some kind of sensor?

 

[Mike - K8LH]

Can you set -vRef at 2900-mV and +vRef at 3923-mV to provide 1-mV resolution on the ADC result (voltage = ADC result 0..1023 + 2900)?

Hi Mike, I think everyone else is trying to avoid that suggestion lol, the next question would be....how do I get an accurate reference for 3923 mV .

I was just asking if it was possible. I've never used anything other than ground for -Vref.

If you did want to use those references, couldn't you route the 4.096v FVR (Fixed Voltage Reference) output to an I/O pin and attach a couple ten-turn pots between that pin and ground with the wipers connected to the -Vref and +Vref input pins?

That's probably quite a bit of work to get 1-mV resolution when you consider that grandad's suggestion for using the 4.096v FVR for +Vref will provide 4-mV resolution for a 0..4096-mV input when using a 10-bit ADC.

Cheerful regards, Mike

 

[large_ghostman]

Hi Mike I will read some sheets, I think if it can be used like this then I for one have overlooked a very useful feature for a long time!!
How many threads asking about 1mV resolution etc etc. I might play with this later.

Still in the back of my mind is a vague memory of some limitation or other with the -Ref, so rather than guess I will go read :

 

[Mike - K8LH]

Thanks...

 

[atferrari]

Thanks to RonS, Mike and GD for replying.

Good to know I was not that much out of track.

After reading all datasheets, the best micro I got here is the 18F4431 with a minimum delta Vref=1,5V (but no FVR on board).

 

[large_ghostman]

Thanks to RonS, Mike and GD for replying.

Good to know I was not that much out of track.

After reading all datasheets, the best micro I got here is the 18F4431 with a minimum delta Vref=1,5V (but no FVR on board).

What am I? Chopped liver? What other chips you got?

 

[JimB]

What am I? Chopped liver?

It can be arranged!
JimB

 

[large_ghostman]

It can be arranged!
JimB

This storm might beat you to it .

 

[granddad]

I gave up trying to get 1 mV res ( 12 bit 4.096v +ref ) as my circuit was tooooooooo noisy ! taking an average of 10 reads gave best result .
I found this a problem PIC 24 A/D SAMPLING REQUIREMENTS " To minimize the effects of pin leakage currents on the accuracy of the A/D Converter, the maximum recomended source impedance, RS, is 2.5 k ohm"

 

[atferrari]

It can be arranged!
JimB

By that, Jim, you mean that I could use an external voltage reference?

 

[large_ghostman]

LOLOL ok you been found ot! I am on ignore lmao, thats why he dosnt see what Jim is saying . Jim your going to have to explain he has been found out lol. I wish the software told you who was ignoring you, I have read data sheet after data sheet for him and couldnt have posted it anyway as he wouldnt of seen it .

 

[ronsimpson]

you mean that I could use an external voltage reference?

Typically:
-Vref is connected to ground
+Vref can be connected to +supply but a +5V supply is not as good as a real reference
+Vref can be connected to a reference like 4.024V or 2.5V or......

SO

We are saying that +Vref can go to +5V and -Vref can go to a reference just as well.

Vsup is you 5V.
Rsup could be 1k to 2k. Needs to have 1 to 10mA of current.
R1 and R2 should be in the 10k range.
Vo is set by the formula. If R1=10k and R2=10k then Vo=2.5V Increasing R1 will increase the voltage.
Cl could be 0.1uF
TL431 is a little "transistor" looking thing. Three legs.

 

[atferrari]

Thanks Ron for the details

TL431 is a little "transistor" looking thing. Three legs.

Maybe around 20 years ago I've seen one literally melting on a protoboard. Brave, it worked until the last moment.