Chapter 8 Tutorial

Measurement Fundamentals

Sources of Error in DC Voltage Measurements

Common Mode Rejection Ideally, the internal DMM is completely isolated from earth-referenced circuits. However, there is finite resistance and capacitance between the input LO terminal and earth ground. If the input terminals are both driven by an earth-referenced signal (Vf) then a current will flow through RS and create a voltage drop VL as shown below.

Any resulting voltage (VL) will appear as an input to the internal DMM. As the value of RS approaches zero, so does the error. Additionally, if Vf is at the power line frequency (50 Hz or 60 Hz), the noise can be greatly reduced by setting the internal DMM’s integration time to 1 PLC

or greater (see page 103 for a discussion of integration time).

HI

Vtest

 

 

 

 

 

DMM

 

 

 

 

 

 

 

RS

LO + VL -

≈ Ci

250 pF

Ri

Vf

 

 

 

 

 

 

 

 

 

>10 GΩ

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Where:

Vf = Common mode float voltage

Rs = LO lead resistance

Ri = Isolation resistance

Ci = Isolation capacitance

Zi = Parallel impedance of Ri + Ci

Vf x RS

8

Error (VL) =

RS + Z

355