voltmeter with adc0831

[kiranswaps]

adc0831

dear freinds
i want to design a voltmeter of range 0 to 0.1 volts using 8 bit adc (ADC0831) and atmel microcontroller 8051 with a full scale resolution .

00000000b should represent 0volts
11111111b should represent 0.1volts

please help me how to go about this

thanks
kiran

 

[IanP]

adc 0831

First you will have to amplify input signal from 0-0.1 to 0-5V.
To do this you will need an amplifier (opamp based) that will have gain=50.
Here is a link to the ADC0831 data sheet where you can find an example code written for 8048. It is an "older" version of 8051 and it is very easy to translate ths code directly to 8051..
http://info.hobbyengineering.com/specs/NATSEMI-ADC0831.pdf

Good luck ..

 

[silvio]

adc0831 routine 8051

Since the voltage reference input can be adjusted to allow encoding any smaller analog voltage span to the full 8 bits of resolution, you can have one of 0,1V. However, particular care must be taken with regard to noise pickup, circuit layout and system error voltage sources when operating with a reduced span (0,1V) due to the increased sensitivity of the converter (1 LSB equals VREF/256 = 0,39 mV). Thus linearity error increases and unadjusted offset error as well.

Choosing to amplify the input signal by 50 it's by far the strong recommended approach to decrease these errors.
For single-supply operational amplifier the suitable for your application is one that
provides rail-to-rail operation on the input.

 

[kiranswaps]

ca3130 alternative

Thanks for the suggestions. Actually i tried the same thing from a book by "jan axelson", below is the diagram of the same thing. But it didn't worked. actually i'm new to op-amps. please explain me in detail. one thing more one of my friend told me a op-amp wouldn't give full linear response? is it true? please help me out, & which op-amp should i use.

 

[IanP]

8051 voltmeter

For this application you should use precision instrumentation opamp, such as of AD-series from Analog Devices (examples here: **broken link removed**) or Linear Technologies. They will provide very linear response, but the supply voltage has to be higher than 5V ..
As a cheaper alternative you can have a look at CA3130; it is very good at the 0V region and again, the single supply voltage has to be around 7-8Vdc to provide linear response between 0 -5V output ..
After all you have only 8-bit resolution so 10mV ( 0-5V range) this or that way really doesn't matter much ..

 

[wizpic]

adc0831 circuit

why not use a pic they have 10bit resolution shuch as 16f877 or 18f452 which can be run at 40mhz it was just a thought

wizpic