ADC onboard or external...Pros and Cons

[kiranpatil]

Hi,

I am designing a data collector and monitoring unit.
I need to monitored around 24 channels. I am in dilema over selecting a MCU.

Should I go for the MCU which is having 24 onboard ADC channels or I should think of having an external MUX and then less number of channels in MCU?

What will be the disadvantages of having external MUX for ADC?

 

[Ian Rogers]

Its all down to speed and precision really... Most MCU have 10 bit ADC's... If 10 bit will suffice, then the MCU can be set up to sample up to 16khz (DSP's can do a lot more).

As this is in the 8051 section I'm assuming an 8 bit micro is to be used (I haven't seen one with 24 ADC's onboard, but some have quite a few so I'll take your word).

If you use an external multiplexed device you'll need to get the data from each one.... My guess would be serially ( SPI )... Then sampling would be slower ( unless you intend to use flash) but can be more resolute.

So the question is... How resolute do you need your results? How fast do you need to sample?

Example..... My data is quite slow to change so I sample 24 times in a session, then about 8 times a second... 24x oversampling gives me adequate resolution, and displaying quick enough for the human eye....

 

[3v0]

Ian is correct.

In the PIC world you have one or two onboard ADC's and a onboard mux so each ADC can service serveral pins/channels.

Unless you need faster conversions or more ADC bits the onboard ADC's will simplify the hardware. Going with a uC with 2 onboard ADCs will let you do two conversions at once.

 

[Sceadwian]

24 signals doesn't tell us much, how much accuracy and speed do you need? The 10bit ADC that most micro's have will never get 10bits at their highest sampling rate generally speaking ADC clock has to be run slow and the chip has to be put into a sleep mode during the conversion for best accuracy.

 

[kiranpatil]

Few more details

Adding few percise details:
First of all my appologies that I have wrongly put this question under 8051 section.
Actually I am consider MCU like LPC23xx/FreeScale Kinetics/STM32. I want to run embedded linux on it.

My issue with these MCU is less number of ADC pins on-board.
I do have around 24 inputs to be measured. I need accuracy of 0.01% in my measurement.
I will be taking a reading/measurement every 2-3 minutes.

Hene I was exploring options of external ADC but not sure of what design contrainsts it will bring in.

 

[Sceadwian]

.01% doesn't mean anything without a full scale reference... What is the possible range of your measurement?

Can the measurements occur asymmetrically or do they need to be read at the exact same moment? Based on the time constraints you've given so far you only need a single ADC port (of an as yet unable to determine accuracy) and an analog multiplexer.

The better you can define the exact signal you're trying to measure the range you're trying to measure it and exactly what this is for the better a proper response can be given.

Don't be afraid to bore us! The more details you can give the better.