The Pyxis controller board is designed around an ATMega128 8-bit micro controller operating at 16 MHz. This micro controller executes most instructions in a single clock cycle, has a built in 10-bit ADC, 128 kb of electrically erasable program memory, 4 kb of EEPROM data memory and 4 kb of internal RAM. A block diagram showing the architecture of the controller board is shown below
Video sampling circuit
The video signal is converted to a 16-bit digital word using an Analog Devices AD7671, 1.2 MHz, 16-bit sampling ADC. The ADC is connected in "byte swap" configuration, so that first the high byte is read followed by the low byte. The data bytes are available to the MCU on the 8 bit-bus, or can be piped directly to the USB output FIFO. The video signal from the CCD board is negative due to various optimizations having to do with the op-amp configuration in the output chain (specifically, the video signal is always placed on the + input of the op-amps due to the requirement for very low input impedance). This signal is inverted and buffered by a fast bipolar op-amp with good CMMR (AD8021) which is configured in differential configuration. The ground from the coaxial cable is connected to the analog ground plane through a 10 Ohm resistor (R22) and input on the one arm of the differential amplifier so that noise common to the ground and signal lines is attenuated. The signal from the two CCDs is routed to the ADC input using an ADG453 analog switch (4 Ohm "on" resistance). A pair of diodes (D3) clamp the input between 1V and -13V to prevent the input range of the ADC from being exceeded. A separate 5V supply is used to power the analog section of the AD8021 to minimize noise. The conversion noise is approximately 1.5 to 2 ADU RMS for a -5V source.
Temperature sensor buffer circuit
An LT1013 op-amp is used to buffer the temperature signals from the two cameras before it is applied to the ADC input pins of the ATMega128. This circuit uses the same ground-sensing arrangement used by the video section. A pair of diodes on each output ensures that the signal remains between -1V and +6V.
Other digital circuits
The remaining digital section of the board is either self-explanatory or in the case of the FT-245BM, is specified by FTDI's data sheet. The only modification made to FTDI's circuit was to connect the ground line of the USB port to Pyxis's system ground through a 50 Ohm resistor. This was found necessary to prevent ground loop noise from appearing in the images when the PC was powered through the same 12V supply as the camera. The additional noise caused by the ground loop was only about 2-3e-, but was particularly objectionable due to its periodic (as opposed to random) appearance in the image.