Stanford Research Systems SR780, Operating Manual And Programming Reference

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2-58 Input Connections
SR780 Network Signal Analyzer
Input Connections
In order to achieve the best accuracy for a given measurement, care must be taken to
minimize the various noise sources which can be found in the laboratory. With intrinsic
noise (Johnson noise, 1/f noise or input noise), the signal source must be designed with
these noise sources in mind. These noise sources are present regardless of the input
connections. The effect of noise sources in the laboratory (such as motors, signal
generators, etc.) and the problem of differential grounds between the signal source and
the analyzer can be minimized by careful input connections.
There are two basic methods for connecting a voltage signal to the analyzer - the single-
ended connection is more convenient while the differential connection eliminates
spurious pick-up more effectively.
Single-Ended Connection (A)
In the first method, the analyzer uses the A input in a single-ended mode. The analyzer
detects the signal as the voltage between the center and outer conductors of the A input
only. The analyzer does not force the shield of the A cable to ground, rather it is
internally connected to the analyzer's ground via a resistor. The value of this resistor is
selected by the user. Float uses 1 M
Ω
and Ground uses 50
Ω
. This avoids ground loop
problems between the signal source and the analyzer due to differing ground potentials.
The analyzer lets the shield 'quasi-float' in order to sense the source ground. In general, if
the source is floating, use a Grounded input. If the source is grounded, use a Floating
input. Do not use a Floating input with a floating source since the shield is simply a noise
antenna in this case. Noise pickup on the shield alone will appear as noise to the analyzer.
Common mode noise, which appears on both the center and shield, is rejected by the
common mode rejection (CMR) of the analyzer input, but noise on the shield only is not
rejected at all.
Differential Connection (A-B)
The second method of connection is the differential mode. The analyzer measures the
voltage difference between the center conductors of the A and B inputs. Both of the
signal connections are shielded from spurious pick-up. Noise pickup on the shields does
not translate into signal noise since the shields are ignored. The shields in this case are
grounded by 50
Ω
since they are being used as signal shields.
When using two cables, it is important that both cables travel the same path between the
signal source and the analyzer. Specifically, there should not be a large loop area
enclosed by the two cables. Large loop areas are susceptible to magnetic pickup.
Common Mode Signals
Common mode signals are those signals which appear equally on both center and shield
(A) or both A and B (A-B). With either connection scheme, it is important to minimize
both the common mode noise and the common mode signal. If the signal source floats at
a non-zero potential, the signal which appears on both the A and B inputs will not be
perfectly canceled. The common mode rejection ratio (CMRR) specifies the degree of
cancellation. For low frequencies, the CMRR of 90 dB indicates that the common mode
signal is canceled to 1 part in 30,000. Even with a CMRR of 90 dB, a 1 V common mode
signal behaves like a 3 µV differential signal! The CMRR decreases by about 6
dB/octave (20 dB/decade) starting at around 1 kHz.