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1mA Constant Current Source PSU for Hall Bar Probe U2 generates a fixed voltage of between 2.46V & 2.76V adjustable via R2 which enables the output current to be adjusted accurately to 1mA. This voltage is then divided down by R17 & R9 to provide the reference voltage for the error amp U7 which maintains the voltage drop across R19 (100 ohms 0.1%) equal to the reference voltage at U7s pin 3 and hence the output current is kept constant. The compliance voltage of the device is 10V therefore a 1mA current can be maintained through a maximum Hall Bar resistance of 10K ohms. The current can be measured in line or the voltage between I- and the casing of either BNC connector will be equal to I x 100 ohms i.e. 100mV for 1mA. The output of the unit is not referenced to mains earth unless the GND switch is in the ON position. The green terminal adjacent to the internal mains connector can be used to ground the Hall Bar cable screen if desired. The Hall voltage is output via the simple filter ( composed of R10,R11,C1,C2,and C4,C5,C6 & C8 if desired ) to the differential BNC connectors on the front panel the screens of these connectors are not referenced to mains earth unless the GND switch is in the ON position.