AD - Converter calculations ?

**NetDevil** · 14th May 2009 05:05 PM

Hey guys i was wondering if you could help me out with my calculations.

I'm currently working on a project were we use an ultrasonic sensor to measure distance and later will convert to speed. the output from this is equal to 9.8 mV per inch.

We are using a 10bit AD-converter and we have chose to only read the 8 MSB. and now comes my problem: how do i calculate the output from the ADC to the actual distance?

my reference voltage is 2.5V

I really hope you can help me because i'm stuck(Brain freeze).

**Nigel Goodwin** · 14th May 2009 05:44 PM

You've just said that the sensor provides 9.8mV per inch, so 98mV will be 10 inches, 980mV will be 100 inches and so on.

I suspect you will need to amplify the sensor output to give the best resolution you want it amplifying to 2.5V at the maximum range you want.

**NetDevil** · 14th May 2009 05:49 PM

Originally Posted by Nigel Goodwin

You've just said that the sensor provides 9.8mV per inch, so 98mV will be 10 inches, 980mV will be 100 inches and so on.

I suspect you will need to amplify the sensor output to give the best resolution you want it amplifying to 2.5V at the maximum range you want.

Yes but i want to use the AD-converter result i.e. 37(decimal) at aprox 82 inches - to later on convert to speed. i guess my problem is that i don't understand the result from the conversion.

**blueroomelectronics** · 14th May 2009 05:57 PM

Which compiler are you using?

**NetDevil** · 14th May 2009 05:59 PM

Which compiler are you using?

Im using hitech and im programming pic16f887

**Nigel Goodwin** · 14th May 2009 06:01 PM

Originally Posted by NetDevil

Yes but i want to use the AD-converter result i.e. 37(decimal) at aprox 82 inches - to later on convert to speed. i guess my problem is that i don't understand the result from the conversion.

Have a read of my analogue PIC turopial which explains it a bit.

**NetDevil** · 14th May 2009 06:10 PM

Originally Posted by Nigel Goodwin

Have a read of my analogue PIC turopial which explains it a bit.

Thank you but i think i understand the basict with the resulution and such. but its just my result that makes me doubt.

Can it be because i don't take account the two LSB? i mean when measuring at aprox 1/3 of its range it gave me 37 and i just don't understand why when i have a 10 bits AD-Converter i mean shouldn't it give me 1/3 of 1024?.

**blueroomelectronics** · 14th May 2009 06:18 PM

What's the maximum range you'd like to measure? Keep in mind the sensor will have limited range.

**NetDevil** · 14th May 2009 06:20 PM

Originally Posted by blueroomelectronics

What's the maximum range you'd like to measure? Keep in mind the sensor will have limited range.

The sensors max range is 254 inches or 6,45 meters.

i'm measuring up to three meters

**blueroomelectronics** · 14th May 2009 06:56 PM

If you can trim your +VREF to 2.602V then your sensor should give you 1" per step at 8bits.

255 / 0.098 = 2.602

**NetDevil** · 14th May 2009 07:41 PM

Originally Posted by blueroomelectronics

If you can trim your +VREF to 2.602V then your sensor should give you 1" per step at 8bits.

255 / 0.098 = 2.602

ah.. thank you

that makes it a lot easier !

**blueroomelectronics** · 14th May 2009 08:27 PM

My math was way off and zero counts so

254" = 2.4892V max

Adjust your VREF to 9.8mV * 256 = 2.509V

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