Decoding negative millivolts counts in ADS1241 24 bit ADC

Question

I am using an ADS1241 to measure thermocouple signals.

I have applied a millivolt signal differentially to channel AIN4 and AIN5 using a stable and accurate millivolt source.

The thermocouple signal ranges from -8.9 mV to 70 mV. I want to cover the whole millivolt range using this ADC.

I have set the PGA to 1.

How can I measure the negative millivolts?

I applied -10mV to the input and getting counts FF47D3 in hex. The reference is 1.235V and the format is bipolar. I am not able to decode these ADC counts.

Datasheet for the ADS1241.

Answer 1

I've never done this or worked with 24-bit numbers and a quick search reveals little. The datasheet is quiet on the topic too.

I assume that the ADC would count down past zero in the sequence

0000 0000  0000 0000  0000 0011 = 3
0000 0000  0000 0000  0000 0010 = 2
0000 0000  0000 0000  0000 0001 = 1
0000 0000  0000 0000  0000 0000 = 0
ffff ffff  ffff ffff  ffff ffff =-1

Wikipedia's gives the following example:

The following Python code shows a simple function which will convert an unsigned input integer to a two's complement signed integer using the above logic with bitwise operators:

def twos_complement(input_value, num_bits):
    '''Calculates a two's complement integer from the given input value's bits'''
    mask = 2**(num_bits - 1)
    return -(input_value & mask) + (input_value & ~mask)

Trying this in a Python console ...

mask = 2**(24 - 1)
decimal = -(0xff47d3 & mask) + (0xff47d3 & ~mask)
print decimal
print decimal * 1.235 / 2**23
>>> 
-47149
-0.00694143950939
>>>

... which is about 7 mV.

---

A simpler solution depending on your microcontroller would be to left shift the number by four bits. This will preserve the two's complement but make it available to a 32-bit calculator.

0xff47d3 << 4  // results in 0xff47d300 = -12070144
Oxff47d300 / 256 = -47149

which is the same number we got using the Python method.

This seems a little odd in that the 4-bit left shift is usually considered a x 256 multiplication and then we're following it by a / 256 division so why bother? The answer is that the left shift preserves the two's compliment and the sign of the number for use by a 32-bit arithmetic logic unit. The division can then proceed correctly.