Important CCD voltages are set by 8 bit AD7228 Digital-to-Analog Converters (DACs) made by Analog Devices.
Table 6: Digital-to-Analog Convertor Logical Channel Assignments
The 32 logical DAC channels are listed in Table 6, together with the conversion factors from uploaded DN (8 bits) to voltage applied to the CCD. This table gives the DAC channels as they are addressed by the SETDAC serial command (SC26). Nominal settings for these DAC channels are given in Table 7 for the CUBIC flight CCDs.
Table 7: Digital-to-Analog Convertor Nominal Settings
The columns labeled ``Default'' give the default values that should be loaded by the flight software. The columns labeled ``Updated'' give updated values loaded by serial command for improved performance.
We use four 8 bit DACs with 8 channels each. However, the address decoding hardware is configured so that they appear to be two 8 channel by 16 bit devices. Therefore, the software interface loads pairs of DAC channels. The actual DAC hardware addressing is shown in Table 8.
Table 8: Digital-to-Analog Convertor Hardware Channel Assignments
The DAC channels can be tested/calibrated using the RAMPDACS serial command (SC36). This command ramps all of the DAC channels from 0 to 255, collecting a complete housekeeping measurement at each DAC DN level to check for linearity, correct calibration, missing codes, etc. The output data are written into the Science Data RAM using a modified Stripchart HK Frame mode described in § 4.6.6. This calibration mode is disabled in the flight code to prevent damage to the CCDs.