Help with converting 10 bit adc to 12 bit dac output

diode_blade · Posted: Tue Jan 20, 2015 7:40 am

CCS Ver 5.020
PIC 18f4431
Hardware board: Pic Millenium prototype board (from the year 2000!)
Breadboard: with 2 low pass filter opamp circuits into an AD626 differential amplifier circuit-output, then into the 18f4431 a/d.
DAC USED is MAX538, Vref to dac set at 2 volts.

The inputs (they also drive a center zero 100-0-100 uA meter) to the low pass filters are differential inputs from a pulse inductor metal detector circuit (no common ground between detector circuit and my PIC and other circuitry)), I use the DAC o/p to feed back to the met detector board to re-zero it after metal has passed over the head, basically i am trying to compensate for drift on the detector board and in the coil head.

Algorithm

Mike Walne · Joined: 19 Feb 2004 Posts: 1785 Location: Boston Spa UK

You've had no response.

I assume it's because none of us can figure out what you are trying to do.

Have another go.

Mike

rikotech8 · Joined: 10 Dec 2011 Posts: 376 Location: Sofiq,Bulgariq

12bit_dac_val = 10bit_adc_val << 2;
will give you 12bit value with 10bit resolution.
_________________
A person who never made a mistake never tried anything new.

Ttelmah · Joined: 11 Mar 2010 Posts: 19375

Very much agreed with Mike. He is wanting to do more than just convert from 12 to 10bit, but it is not at all plain 'what'. He talks about voltage ranges, and midpoints, but then doesn't explain whether these are source, or destination.
I think if he drew out his actual endpoints and midpoints for both source and destination on a bit of paper as a graph, with the ranges over which these can move, the transfer function would become visible, as well as what has to adjust to deal with the shift.

diode_blade · Posted: Fri Jan 30, 2015 10:07 am

Thanks Ttlemah,Mike and Riko for the reply, keeping it as simple as possible,
take a value from the ADC (call it signal) and store it as center_value, if the signal drifts either way from the center value find the difference and calculate a value to output to the DAC to bring the signal back to the center value, sort of auto zero
ADC 18f4431 10 bit
Max538 dac 12 bit

what I have managed to do is this and it works reasonably well

Mike Walne · Joined: 19 Feb 2004 Posts: 1785 Location: Boston Spa UK

Assuming you know the OPA gain and ADC & DAC transfer functions you should be able calculate a new DAC value so as to bring the output back to its centre value.

I don't see why you need to deal with +ve and -ve errors separately.
You're calculating an error as a +ve value in both cases.
In one case you add an offset, in the other you subtract.

Surely its simpler to get the error as either a +ve or -ve value and deal with only one process. Like Ttelmah says, it's a pencil and paper job.

Mike