Design Report
Load Control Analog Board
Tethys/LRAUV

Ed Mellinger
6 Aug 2010

The 'Load Control Analog' board is a high-resolution A/D converter and associated circuits for driving a bridge-type pressure sensor.  It occupies a standard Load Control Board in lieu of the Serial I/O section, and is intended  to provide a low power (and redundant) method for monitoring a vehicle depth sensor when the CTD payload is turned off (or inoperative).

Requested performance was as follows:

	Resolution: 	1 part in 60,000 (0.5 cm in 300 m) (16.7 ppm) (16 bits)
	Accuracy: 	1 part in 2,000 (.05%)
	Speed:	return sample in 10 msec or less; update at 10 Hz rate.

The intended pressure sensor is the Keller PA-6S, a piezoresistive type.   As is typical for the type, this sensor is specified by the manufacturer to be driven with a constant current, which gives a first-order cancellation of the temperature drifts of bridge resistance and sensitivity.  Keller also specifies external resistors to be included in-circuit for compensation of offset error and span drift with temperature.