How do I determine the value for a shunt reference\s series resistor?
For a shunt reference to provide a stable voltage, a certain amount of current MUST be flowing THROUGH the reference. This current is determined by a resistor between the power source and the reference input. For example, for the LM385, there should be 50uA to 1mA of current flowing through the LM385. 100uA is the current at which all the specifications are made, and is a good operating current for the reference. To figure the value of the resistor Rs, you must also make allowances for any current shunted away from the reference by the circuit connected to the reference output. The equation is: Rs = (Vsupply – Vreference) / (Iload + Ireference). For a 1.2V reference with a 5V supply, and no significant load, Rs = (5V – 1.24V) / 100uA = 37.6kohm. The actual value of the resistor is not very critical. 10k to 39k would work fine. To determine load current, be sure to check the “Reference Current” (Iref) spec, or calculate it from the “reference input resistance” spec, of your A/D or D/A c
