Keysight U2741A Guide D'utilisation page 57

Other Primary Measurement Functions
Frequency measurement errors
The U2741A uses a reciprocal counting technique to measure frequency. This
method generates a constant measurement resolution for any input frequency. All
frequency counters are susceptible to errors when measuring low-voltage,
low-frequency signals. The effects of both internal noise and external noise pickup
are critical when measuring "slow" signals. The error is inversely proportional to
frequency. Measurement errors also occur if you attempt to measure the
frequency of an input following a DC offset voltage change and therefore, you
must allow the digital multimeter's input to fully settle before making frequency
measurements.
DC current measurements
When you connect the digital multimeter in series with a test circuit to measure
current, a measurement error is introduced. The error is caused by the digital
multimeter's series burden voltage. A voltage is developed across the wiring
resistance and current shunt resistance of the digital multimeter, as shown below.
Keysight U2741A User's Guide
Measurement Tutorial 3
57