Agilent Technologies Agilent 33500 Series, User Manual

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Chapter 5 Tutorial
Ground Loops
5
Quantization Noise Finite resolution in the waveform DAC causes
voltage quantization errors. Assuming the errors are uniformly
distributed over a range of ±0.5 least-significant bit, the equivalent noise
level for standard waveforms is approximately -95 dBc. At this level,
other sources of noise in the instrument dominate. Quantization noise
can be of concern, though, in arbitrary waveforms that do not use the
whole range of DAC codes (-32767 to +32767). Scale arbitrary waveforms
to use the entire range, if possible.
Ground Loops
The signal-generation portion of the 33500 Series is isolated from chassis
(earth) ground. This isolation helps eliminate ground loops in your
system and also allows you to reference the output signal to voltages
other than ground. The illustration below shows the waveform generator
connected to a load through a coaxial cable. Any difference in ground
potentials (V
GND
) will tend to drive current I
GND
through the shield of
the cable, thus causing a voltage drop due to the shield’s impedance
(Z
SHIELD
). This voltage (I
GND
x Z
SHIELD
) appears as an error in the load
voltage. However, since the instrument is isolated, there is a high series
impedance (typically >1 M
 . in parallel with 50 nF) to oppose the flow of
I
GND
and thereby minimize this effect.