CODEWITHSUNDEEP
matlabcastingserial-portbit-shiftadc
markos14
markos14

Reputation: 88

Convert 24-bit ADC serial read data from 3-byte format to signed integer (int32) in Matlab

I am receiving EEG data from a 24 bit ADC over serial. The ADC data is transmitting in 3 bytes from MSB to LSB. The full packet is 21 bytes:

The first byte is the start byte - 0xFF (255 in decimal) Then packet number byte. Then the next 3 bytes are the 24 bit ADC value broken into MSB LSB2 LSB1 I can parse the data fine, but re-constructing a 2's complement signed int32 number is causing issues. The values I am getting out certainly don't reflect what the ADC should be giving out.

Below are the lines to read and parse the 504 samples (which gives me 24 ADC values (504samples/21bytes = 24 values)). I have tried uint8 instead of uchar with similar results (when I try int8 I get a invalid specified precision error).

comEEGSMT = serial(com,'BaudRate',3000000);
fopen(comEEGSMT);
rawData(1:504) = fread(comEEGSMT, 504, 'uchar');
fclose(comEEGSMT);
startPackets = find(rawData == 255);
bytes = rawData([startpackets+2 startpackets+3 startpackets+4]);

I have tried the following method to reconstruct the value:

ADC_value = bytes(:,1)*256^2 + bytes(:,2)*256 + bytes(:,3);

and the following line is the formula to convert the above number to volts:

ADC_value_volts = ADC_value*(5/3)*(1/(2^32));

The values are in the range of 4000 - 8000 microvolts with large jumps in value. The values SHOULD be in the range of 200 - 600 microvolts with small changes.

I have found other questions relating to similar issues, but have had no success trying the proposed solutions such as in the link below:

https://uk.mathworks.com/matlabcentral/answers/137965-concatenate-3-bytes-array-of-real-time-serial-data-into-single-precision

Any help would be very much appreciated as I've been stuck on this for quite long.

Thanks Mark

Upvotes: 0

Views: 1800

Answers (1)

Yoan
Yoan

Reputation: 161

Starting with ADC_value as int32 with value 0, then:

ADC_value |= MSB << 16;
ADC_value |= LSB2 << 8;
ADC_value |= LSB1;

And then, to find out the corresponding volts value, supposing your ADC has a reference voltage VREF, in volts (e.g. 5.0V): 

ADC_value_volts = (ADC_value * VREF)/2^24

since your converter is 24 bits, not 32. Note the above expressions are in C language equivalent, not Matlab.

EDIT:

The ADC data sheet tell us the PGA gain can be set for the following values: 1, 2, 4, 6, 8, 12, 24, one value at the time for each channel. The FSR (full scale range) of measurement is: (2*VREF)/Gain = 5/3, for Gain=6, (eq.(5) page 23) so this must be accounted for in expression computing the volts values. (these can be verified if you have access to the hardware and can make some measurements).

Data resulted from ADC is already in two's complement, binary form, 24 bits. The weird thing is the data sheet counts bits starting with 1, not 0, so this is why shifting with "17" instead 16 - this is in fact 16 for coding. (revealed in fig 47, page 42).

So the computing formula of ADC_value_volts should be:

ADC_value_volts = (AC_value * FSR/(2^23))/3 (1LSB=FSR/(2^23), pg.37)

If some other calculations/modifications from original, them these must be explained by provider. If the provider is not friendly, worth to be changed...

Upvotes: 1

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