subtracting 2 integer values

[Raymondkoehler]

Hi, i'm Raymond from the Netherlands and currently programming in VHDL.
I think i do not quite understand what happends if i subtract two integer values.

What i have is 2 adc's and 1 dac.
1 adc has a 14bit 2's complementcode as a output.
1 adc has a 8 bit offsetbinarycode as a output.
The dac has a 14bit offsetbinarycode as a input.
Now want to subtract both adc values and put the result on the dac. I did the following:

I translated the 2's complementvalue of the 14bits adc into a offsetbinary value by inverting the msb-bit.(this is possible and correct). Then i converted it into a integer.

I translated the offsetbinary value from the 8 bit adc into a 14bit value by converting it into a integer and multiplying that with "64". Now i subtract both signals (as a integer) and reconvert that value into a std_logic_vector again to put it on the dac.

Compleet rubbish is comming out of the dac. IT really seems like suddenly there is a offset in the dac output if i pass sertain values.
It is not the blame of the conversionfunctions caus i tested them independently (without subtracting), and then it is working perfectly. So only if i use the subtraction it is not working.
I suspect the signbit is the problem here, or something with a underflow.
Can somebody direct me in this? I'm reaching the bottom off my idea's
Thanks in advance,

Raymond

 

[Raymondkoehler]

ok, got it fixed finally. By adding 16384 to the subtracting integer it will never drop below zero so no negative signals are being generated or calculated with.
Problem solved

Laters,

Raymond