Abstract: Disclosed are compounds having the formula wherein R1 is hydrogen, an alkyl group, an aryl group, a cycloalkyl group, an alkenyl group, an alkynyl group, an aralkyl group, or ?O, R2 is an alkyl group, an aryl group, a cycloalkyl group, an alkenyl group, an alkynyl group, or an aralkyl group, or the pharmaceutically acceptable salt or ester thereof. Also disclosed are methods of using the compounds.

Abstract: A combined voltage and current source for measurement systems including a single reference amplifier provided at the front end which receives a reference voltage for determining the level of the voltage or current source signal supplied at the output. CMOS switches or similar logic selects between current and voltage portions of the combined circuit. If the voltage function is selected, the reference amplifier controls a voltage regulator to maintain the output voltage approximately equal to the reference voltage applied at the input. If the current function is selected, the reference amplifier is configured as a voltage to current converter to establish a current through a precision resistor. A voltage follower circuit provides the current source signal referenced to a supply voltage for providing a positive current. In this manner, a single reference amplifier is used at the front end rather than two separate reference amplifiers provided in prior art.