SOME NEW INTERESTING INSIGHT INTO THE CCD TEMP ISSUE
@reyalp
Perhaps you may recall a discussion we had about the discrepancy between the actual CCD temp I was measuring and the one reported on the OSD. At the time while the CCD was hanging in the air I found that difference to be 10-12C, higher on the OSD than measured. Thus with the matchbox-S90 (instrument 101a) sitting on the bench I would see around 40C to 45C at room temp on the OSD. You claimed the OSD value to be accurate and the discrepancy to be thermal resistance between the CCD surface (you assumed a thermistor on the silicon surface of the CCD die) and its ceramic package. While theoretically true (specified as degC/W in power designs with an infinite heatsink), such a large difference can only be seen when a high-power transistor is overdriven way beyond its capacity for its heatsink to bleed out the heat effectively. The case of the camera CCD is the reverse, that is, it is ultra low power and there is no effective heatsink. So because of its low power it takes several minutes for the CCD to reach a stable temp while hanging in air, and if that kind of difference were real, the CCD would burn up in a bright flash!
The matchbox-S90 is now mounted on its new machined heatsink assembly with a precision TEC controller capable of 0.01% temperature regulation (it even looks beautiful in its ugliness). Although mounted on the TEC, at present the CCD is not yet in its hermetic cap (ie exposed to air) but cooling it down to near dew point is possible. Thus using both an IR thermometer and a contact thermometer that agree, the CCD is at +18C, rock stable.
However, the OSD CCD temp shows +46C // no sense at all. Also, changing CCD temp from +18C to +22C does not change the CCD OSD value. So what's the problem then?
Spraying air gently from a DustOff can across the PCB drops the OSD CCD temp from +46C to +42C in about 10 seconds. Going into PLAY for 1 minute and back to REC, drops the temp by 5C, and the temp goes back to +46C in 2-3 minutes.
The conclusions are that thermal resistance does not account for the discrepancy. Also, there cannot be any thermistor in the CCD die, and, there is no thermistor on the CCD plate and ribbon cable in this camera. Obviously that reading cannot be the actual CCD temp for the S90, and if anything, it is an estimation by FW of some thermistor on the PCB itself. The only schematic available (IXUS 60 - DIGIC II) does show a thermistor mounted on the CCD plate. So the IXUS 60 indeed must report real CCD temp on the OSD in CHDK. I have measured individual chips on the S90 PCB to read as high as +45C. So my guess is that the "CCD temp" OSD value is actually "PCB temp", for the S90. The matchbox-S90 camera is mounted with its exposed PCB facing the heatsink, so I cannot isolate the PCB thermistor position in this configuration. I discovered the problem a few days ago after the whole unit was assembled. When I take it apart again to access the circuit side, I will be able to find and characterize the PCB thermistor accurately.
