How to implement Vref and Vin(-) on an ADC0831 ???

floodhound

I needed some input on how the ADC0831 works when using V- and Vref.
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For example: I was under the impression that if Vref = 3Volts and Vin(-) = 1Volt then I could read a span of 2 volts giving a 0-255 byte value. This is not the case though.
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I found it a bit confusing that on page 197 of “Process and Control guide” that stated if Vin(-) is set to .7V and Vref is set to .5 volts the ADC will convert the range on .7 to 1.2V to a byte value of 0-255.
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Could some one please explain to me in more detail how this is the case. I want to be able to read from 2V to 3V and still get a byte value from 0-255. I am also using the setup like this picture is showing.

RDL2004

Vin(-) is the zero value (bottom end). Vref sets the span above Vin(-) that the ADC reads and divides into 256 parts. So Vin(-) + Vref = top end.

So in the example, Vin(-) is 0.7 and Vref is 0.5 the the ADC reads from .7 to 1.2 (.7 + .5 = 1.2).

Your Vin(+) needs to fall within that span.

So for your 2 volts to 3 volts, you need to set Vin(-) to 2 volts and Vref to 1 volt. then your range is 2 to 3 volts (2 +1=3).

The voltage you want to measure must then be between 2 and 3 volts.

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- Rick

KatyBri

Floodhound-

What temp sensor are you using and when you get your ADC working, could you post your code? Thanks.

floodhound

Thanks again, I kind of figure this to be the case I just wanted it to be clear to me, again thanks a million. Now since I could not sleep last night I have to ask - What is the closest I can set Vref and Vin(-)? At the current I am stated I needed 2 – 3 volts and it is working great, but what if I need to get a better resolution? I have set up the ADC0831 so that I can change the Voltages for “ Vref and Vin(-) “ on the fly so I just need to know my limits. Can I set Vref say to 2.001 and Vin(-) to 2.10 for example?

I am using an LM235 and my code for this bit of hardware is:

[noparse][[/noparse]code]

' {$STAMP BS2sx}
' {$PBASIC 2.5}


'######################################################################################
'######################################################################################

ADC_CS PIN 15
ADC_CL PIN 14
ADC_DO PIN 13
ADC_PW1 PIN 11
ADC_PW2 PIN 12

ADC_RESULT VAR Word

'######################################################################################
'######################################################################################

MAIN:
HIGH 0 'Power LED on this pin
GOSUB SET_AD
GOSUB READ_AD
GOSUB DISPLAY_IT
PAUSE 10

GOTO main

'######################################################################################
'######################################################################################

SET_AD:
'soon add offset allowing for the ability to zoom in on a temp for better resoluition
PWM ADC_PWM, 104, 255 '2v Vin(-)works good for now
PWM ADC_PW2, 75, 255 '1V Vref settings
RETURN


'######################################################################################
'######################################################################################

READ_AD:
LOW ADC_CS
SHIFTIN ADC_DO, ADC_CL, MSBPOST, [noparse][[/noparse]ADC_RESULT\9]
HIGH ADC_CS
RETURN

'######################################################################################
'######################################################################################

DISPLAY_IT:
DEBUG CLS, HOME, "TEMP IS-- " , DEC ADC_RESULT
RETURN

[noparse][[/noparse]\code]

RDL2004

I'm not sure because it's been a while since I read the data sheet, but I think Vin and Vref have to be at least 1 volt apart, so you could get down to 4 millivolt resolution. That's pretty small. If you need more you may need to go to a 10 or 12 bit ADC.

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- Rick

floodhound

I am sorry but i am reading the data sheet and i can not locate the information. I agree though that 1 volt is a great value. Thanks again i just hope you remembered correctly. [noparse]:)[/noparse]

RDL2004

I couldn't find it either. I must have been thinking of how it goes non-linear with a Vref below 1 volt.

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- Rick