User manual

ATHENA CPU User Manual V1.40 Page 61
16. ANALOG CIRCUIT CALIBRATION
Calibration applies only to boards with the analog I/O circuit.
The analog I/O circuit is calibrated during production test prior to shipment. Over time the circuit
may drift slightly. If calibration is desired follow the procedure below. For analog I/O circuit
configuration see page 23.
Four adjustments are possible:
♦ A/D bipolar offset
♦ A/D unipolar offset
♦ A/D full-scale
♦ D/A full-scale
No adjustment for D/A offset is possible.
16.1 A/D bipolar offset
Potentiometer R66, BPOF is used for adjustment. Configure the circuit for Bipolar A/D mode.
Input 0V to any input channel and perform A/D conversions on that channel. The gain setting and
single-ended vs. differential mode do not matter. Adjust R66 until the A/D value is 0. To eliminate
the effects of noise, it is best to take a number of readings and average the values.
16.2 A/D unipolar offset
Potentiometer R67, UNOF is used for adjustment. Configure the circuit for Unipolar A/D mode.
The gain setting and single-ended vs. differential mode do not matter. Input 0V to any input
channel and perform A/D conversions on that channel. Adjust R67 until the A/D value is 0~1. To
eliminate the effects of noise, it is best to take a number of readings and average the values.
16.3 A/D full-scale
Potentiometer R74, ADFS is used for adjustment. Configure the circuit for Bipolar A/D mode
±10V. Input 9.9945V to any input channel and perform A/D conversions on that channel using a
gain setting of 1. Single-ended vs. differential mode does not matter. Adjust R74 until the average
A/D value is 32750. To eliminate the effects of noise, it is best to take a number of readings and
average the values.
Any input voltage and A/D reading near the top of the range (10V) can be used for the calibration
target voltage/reading. The above value is provided as an example.
16.4 D/A full scale
Potentiometer R89, DAFS is used for adjustment. Configure the D/A for 0-10V output range.
Write the output code of 4095 to all four D/A channels. Measure each one and adjust R89 until
the average reading is as close to 9.9976 as possible.