1-2. The Model 1625A Active Current Shunt is 8 precision AC/DC shunt. It incorporates seven decade shunt ranges fro* 200 uA full scale to 100 Amperes. Each shunt range has its own current input terminals and a selector switch provides access to the voltage output terminals of each shunt resistor. A single set of binding posts conveniently provides output voltage to the measuring voltmeter, the performance of the si.unt resistors permits using the 1625A as a laboratory standard as well as a general purpose measuring instrument. 1-3. A. stable precision low noise instrumentation amplifier is incorporated to buffer the voltage output terminals of the shunt resistors* The amplifier has a precisely adjustable gain of 10 so that the output potential from each current shunt lay be used to drive a 200 ohms per volt AC/DC transfer device. Sensed output leads fro* the precision amplifier to the thermal transfer standard maintain accuracy and measuring integrity from dc to well beyond 100 kHz although ac flatness and accuracy is specified to 10 kHz. The precision amplifier permits the shunts to operate at lowest power and also drive an AC/DC transfer device. Internal rechargeable Nicad batteries assure low noise and permit off ground operation. 1-4. The availability of a precision amplifier permits all the current input to the shunt resistors to flow without bypassing measured current to the AC/DC transfer device. This avoids the otherwise limiting condition which restricts passive shunts to a low current of 2.5 mA when used with thermal transfer standards. 1-5. The compliance voltage of the shunts is less than 0,25 volts at full scale for each range except the 100 Ampere range which has a compliance voltage of only 125 mV. Each shunt is a highly stable ac/do resistor connected in a four terminal configuration, A compensated adjustable network controls voltage output to a precise dc and ac current to voltage conversion. The 20 A and 100 A ranges each incorporate a non-inductive shunt resistor made of a specially heat treated alloy developed by Ballantine, These shunts are highly stable with a low temperature coefficient of resistance of nominally +10 ppm per degree centigrade. This.assures rapid settling time with best accuracy and minimum measurement time. 1-6. Forced air cooling is provided by a fan mounted on the rear panel. Outside air is pulled in by the fan, passed over the shunt resistors, and exhausted at the rear panel. DC power is provided for the fan. The fan is operable when using 100, 120, 220, and 240 volts 50 to 400 Hz ac mains power. The internal batteries will provide fan power when operating the 1625A off line. 1-7. The 1625A is housed in a rugged all aluminum enclosure which i$ fully shielded for EHI considerations. The instrument has sturdy die cast end frames, side extrusions, and vinyl clad covers. The enclosure conforms to EIA half rack 5.25 inch high standards and may be rack mounted or used on the lab bench or in portable applications.