Q
The offset of the on-board DAC of the ADUC8XX is 2% (5mV) in the range of 2.5V.
This causes a problem at the customer because they have to set the DAC out to
0-Volts. The FS is not very important. Do you know how they can set the DAC
out to ZERO ?
A
It will not be possible to get zero volts out of the DAC. The DAC has an on
offset of +/-50mV max. If the offset is positive, the output voltage for the
zero code will be roughly equal to the offset voltage (with allowances made for
the headroom of the output amplifier). If the offset is negative, the output
amplifier will saturate to near zero for the zero code. There will be a
"dead-band" around zero where the first few codes after zero will not produce
any response at the output. The output voltage will remain at zero for the
first few codes and then rise when the input code is equivalent to the offset
voltage.
There is no way to determine whether offset will be positive or negative.
If the customer requires a bipolar, true zero output, he can level shift the
output using an opamp circuit. The enclosed circuit will convert the 0 to 2.5V
output from the DAC to a +/-2.5V signal. You can then use software to calibrate
out the offset error. Since you only need 0 to 2.5V output, you will lose one
bit of resolution but I would not expect this to be a problem as the relative
accuracy of the DAC is only +/-3LSB anyway.