Multimeter current measurement: internal resistance or input impedance specification
Burden Voltage


  What is Burden Voltage?
  One thing we rarely look at is the effect we have on a circuit when we attempt to measure the current flowing through it. In this case, the act of inserting the dmm 'A' or 'mA' input shunt in series with a current loop can significantly reduce the actual current flowing in the circuit. If the source voltage in the loop is very high, or the loop is current regulated, then the effect is minimized, but when you are operating in a circuit using a pair of AA batteries in series as a power source, you have only 3 volts to play with, and the shunt of a dmm's mA input can present a significant circuit loading error if you fail to account for it's effect.
  
  So how much effect does the current shunt present? There is a little used specification in the user manual that can help you figure that out – it's called Burden Voltage, and once you know how to use the information, you can make corrections to your measurements to compensate for the effect. 




- Fluke 77IV user manual