ADC output

[jrcfg]

Hey all you electronics guru's, I have a qestion about analog to digital converters....I think I have the concept right but would like someone with more experience to let me know...I have an anolog input that varies from ~2.173V to 2.803V (an accelerometer). I have a 10 bit ADC. My voltage references are Vdd @ 5V and Vss @ 0V. Here's the part I would like some clarification on:

My 10 bit ADC and 5V reference range gives me a resolution of 5/1024 = 4.88mV/Bit. So for every 4.88mV change in my analog input, I should get 1 Bit out, correct? If I got a 10 Bit number of 0111000101 out this would be 2.211V? (4.88mV * 453 ?)

Thanks.

 

[MP1]

You are on the right track, but you do not have to use a 5 volt reference. You can use a different voltage reference that would give you more familiar increments or better resolution.
Look at your full scale. Since your measurement is not going down to zero, you actually are only measuring less than 1 volt. You are wasting the first 2 volts which are always measured and do nothing for you. You could use an offset on the input and then use a 1 volt reference to get much better resolution. For example, with a -2.177 volt offset to bring the ADC input to zero at your lowest measurement, you would be measuring zero to .63 volts. Calculate the resolution for this when using a 1 volt reference.

MP

 

[jrcfg]

WOW! I did not realize a couple of volts would make that much of a difference in resolution. With that 1V ref, the resolution is 0.98mV! Thankyou very much for the reply and the advice. ;D

How would I go about offsetting the input to zero??

 

[MP1]

I would just use an op-amp based summing circuit to add the negative voltage to the input of the ADC. Look for summing amp use for an op-amp. There are quite a few op-amp tutorials on the web if you need to brush up.

MP