When measuring voltages greater than the full-scale input range of a given meter, or when the desired display reading is something other than the direct value of the dc input voltage (Vin), the input must be first attenuated before it is presented to the meter's IN LO (-) and IN HI (+) input terminals. A simple two-resistor voltage divider, similar to that shown in the figure above, will scale the input voltage down to the range of the selected meter. For most applications, a +/-2V input meter ("-1" model) is preferred because its high-impedance input will not adversely affect the voltage developed across R2.
Using either the recommended 909K resistor or a user-defined value for R1, the calculator below will quickly determine the correct value for R2. Please note, this calculator program can only be used with +/- 2V ("-1" input range) dc-voltmeters.
Proper calculator operation depends on Vin meeting the following three conditions:
the desired display readings are directly proportional to Vin; a zero volt Vin will display "000" ("0000" for 4½ digit meters); an increase in Vin will result in a corresponding increase in the display reading. Inputs which include either a zero offset or a negative component ("bipolar") will not produce valid R2 values.
All values for R1 and R2 are entered in K Ohms. As a general rule, R1 and R2 should be precision, ±1%, metal-film resistors with absolute TCR's less than 100ppm/°C for 3½ digit meters and less than 50ppm/°C for 4½ digit meters. See the resistor selection chart below and DMS Application Note 4 and Application Note 14 for more information.