Dewar Monitor System Electronics
Page last updated: September 28, 2011
The Quad Current Source board, as the name might suggest, contains four constant current circuits. Page one of the schematic shows circuits one and 2 while page 2 shows circuits three and four. The 2N4338 JFET establishes a constant current through the 1N914 'temperature' diode housed underneath the CCD inside the dewar. This produces a voltage on the input of the op amp that varies as the temperature of the diode varies. The op amp has a measured gain of about 1.7 and under normal conditions the active range of the temperature seen in a dewar would cause the circuit to vary from about -2V to +2V. However, if there is no diode connected to the circuit the output of the op amp will rise to the positive rail. Because the analog inputs on the RIO are configured for +/-5V operation, we have added the 1N4731 4.3V zener diode to the circuit to limit the output of the circuit. With the exception of the power supply filtering, the connectors, and the zener diodes, this circuit was lifted Mingzhi Wei's UCAM controller schematic.
At the top, left hand corner of the schematic is the input power connector and the onboard filtering of the +/- 12V power. In the left, lower corner is the boards first DB9F input connector. This connector provides wiring for two of the front panel input channels. The two input pins 2 and 1are associated with the two analog grounds on pins 6 and 7. The two CABLE_CONN signals on pins 4 and 5 are associated with the two digital grounds on pins 8 and 9. The digital ground line is wired to the ground plane on the connected dewar's pre amp board on pin 20 and the CABLE_CONN signal is wired to another on pin 16. Thus, by making a connection to a dewar we change the input state to a digital input on the RIO and also provide a ground to turn on the channel's associated red LED on the front of the box.
The second page shows constant current source circuits three and four. These are identical to circuits one and two. This page shows the second DB9F input connector and also shows the output DB25F connector. The signals from this connector wire to the RIO's analog and digital I/O connectors. Note that DB connector pins and signal assignments are tabulated near the connectors for those of us with inexplicably aged eyes!