MIAC Input Resolution

For MIAC users to discuss projects, applications, and any other issues related to the MIAC unit.

Moderators: Benj, Mods

Post Reply
User avatar
Jay Dee
Posts: 398
Joined: Tue Aug 18, 2009 6:42 pm
Has thanked: 121 times
Been thanked: 154 times
Contact:

MIAC Input Resolution

Post by Jay Dee »

Hi guys,
The standard MIAC has an 10bit ADC sampling over the input range of roughly 0-12v.
I require a higher resolution over 0-5V, I was considering hacking the unit so that I could access the PIC18F4455 pins directly, thus get 10bit over 0-5V (using ref+ pin on reference 5v)
Is there an easier way of doing this?
Do any of the MIAC modules offer a higher resolution? or simply over a lower voltage range.

Another option is to use an external ADC chip, but I would still need to break out i2C or SPI.

With the new dsPIC MIAC version on the horizon, what will be the inputs set up as. I think the chip has a 12bit option.

I suspect i may have an easier job taking an dsPIC ECIO and bolting an I2C display and keyboard on. The MIACs are just really quick to implement solutions for customers.
J.

User avatar
LeighM
Matrix Staff
Posts: 2178
Joined: Tue Jan 17, 2012 10:07 am
Has thanked: 481 times
Been thanked: 699 times
Contact:

Re: MIAC Input Resolution

Post by LeighM »

Hi J,

The new MIAC range, including the dsPIC, has the same resolution as the current PIC version due to the use of an MCP3008 so that they are all compatible.

The MIAC Expansion modules (such as the Basic) have Analogue inputs that use a 12 bit ADC over a 0-5V input range.
(The internal processor is a 30F5013)
http://www.matrixtsl.com/webshop/miac-b ... odule.html

These modules are plug and play using the MIAC System project target on Flowcode V7.

Hope that helps,
Leigh

User avatar
Jay Dee
Posts: 398
Joined: Tue Aug 18, 2009 6:42 pm
Has thanked: 121 times
Been thanked: 154 times
Contact:

Re: MIAC Input Resolution

Post by Jay Dee »

Hi Guys,
Looking at the dsPIC MIAC for an upcoming project.
The inputs are 10bit 0-12V, is that from a regulated on board 12V? Or is it just the supply voltage, so I may get input AD variation with dips in the supply?

If I used an advanced MIAC module, to get 12bit 0-5V, what sort of rate is that samples at? or rather what is the fastest I could expect to get it updated to the host MIAC?
Cheers, J.

User avatar
LeighM
Matrix Staff
Posts: 2178
Joined: Tue Jan 17, 2012 10:07 am
Has thanked: 481 times
Been thanked: 699 times
Contact:

Re: MIAC Input Resolution

Post by LeighM »

Hi,
The 0-12V is irrespective of supply voltage.
Using the 8 bit return functions the resolution is 50mV, hence 0 to 12V gives a reading of 0 to 240.
Page 18 of the MIAC datasheet gives more details.

The Advanced module connects via CAN bus so that would be the limiting factor with sample rate.
My guess is that you would get several hundred samples per second, but if you need a more accurate figure I would need to test that in practice.

User avatar
Jay Dee
Posts: 398
Joined: Tue Aug 18, 2009 6:42 pm
Has thanked: 121 times
Been thanked: 154 times
Contact:

Re: MIAC Input Resolution

Post by Jay Dee »

Hi,
So after playing around a bit, here is my notes on getting accurate AV readings into a MIAC.
Testing was done with a MIAC dsPIC, using Analogue Input 1.
Supply to the MIAC can be anywhere across the recommended supply voltage range.

Taking a 8 Bit reading,
255 was reported when a voltage of 13.06 was applied to the input.
200 was reported when a voltage of 10.24 was applied to the input.

[Side note: Haha...10.24! --> 10bit 1024 steps (If your using the 10Bit Input Read Function) ]

For Pots connected directly to the MIAC, you will probably want the best resolution you can sensibly get.
If you only supply 5.00V to your pot, the MIAC is still comparing to its reference 12V so an 8bit Byte reading will only return a maximum value of 97.

For most jobs, where a 8 bit reading is sufficient, a returned value between 0-200 would work quite well.

So my solution is to use an adjustable external linear Voltage regulator, set the output to 10.24V and use this as the supply to all of my Potentiometers.
I then get reliable 0-200 output values over the full sweep of the pot.
Just remember, if you expect drops in supply voltage to your MIAC and external regulator (Car cranking?), the output voltage will probably sag, you might want to consider a little boost converter supplying the reg, so any dips in supply dont effect your Ref voltage.

The great news is that the MIAC internal reference is independent of supply (Obvious but not always the case), I dropped the MIAC's supply below its stated min operating voltage (8V?);
down to 5V actually and the AV inputs still were being referenced correctly. Great!
J.

Post Reply