Applications
Using the 8060A to Measure Extremely Low Currents
3
3-7
3-6. Using the 8060A to Measure Extremely
Low Currents
By using high impedance dc voltage function and high M
Ω
precision
resistors, you can use the 8060A to measure extremely low currents. For
example, if you place a 100 M
Ω
resistor across the 8060A and select the
high impedance dc voltage function and the 200 mV range, the 8060A will
measure a 2 nanoamp (10
-9
A) current with 0.1 picoamp (10
-12
A) resolution.
The error sources with this method of measurement are the combined
accuracy specifications for the voltage range and the resistor, as well as the
8060A input bias current. The input bias current is typically 10 picoamps.
You can measure the input bias current error by removing the test leads and
selecting the normal dc voltage function and the 200 mV range. The number
of digits in the display reading indicates the input bias current in picoamps
(disregard the decimal point). You can correct for the input bias current by
using the relative reference to zero the offset.
The best measurement results will be obtained at ordinary room temperature
with low relative humidity. Be sure to use adequate shielding to prevent
power line or rf interference.
3-7. Making dBm or dBW Measurements with
Other Reference Impedances
The standard power-on reference impedance for 8060A dBm (decibels above
or below one milliwatt) measurements is 600Ω, which is the most common
reference impedance used in the data communications and audio fields.
However, occasionally you might want to make measurements with a
different reference impedance. For instance, the standard rf dBm reference
impedance is 50Ω. Audio power amplifiers use dBW (decibels above or
below one watt) referenced to 2, 4, 8, or 16Ω. The standard method for
making these dBm or dBW measurements is to add or subtract a correction
factor. With the 8060A, however, you can set up any of these reference
impedances with the relative (REL) function.
To change the reference impedance, select the desired dB function and
appropriate range, apply the equivalent voltage level obtained from Table 3-
1 (or from the formulas at the bottom of Table 3-1), and press the REL
button. The 8060A will store the equivalent voltage level, and subsequent dB
measurements will be referenced to the new impedance.
To Next Page
Loading...
