Input Connections - Stanford Research Systems SR810 Manual

SR810 Basics

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 experiment or detector
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 detector and the lock-in can
be minimized by careful input connections.
There are two basic methods for connecting a
voltage signal to the lock-in - the single-ended
connection is more
differential connection eliminates spurious pick-up
more effectively.
Single-Ended Voltage Connection (A)
In the first method, the lock-in uses the A input in a
single-ended mode. The lock-in detects the signal
as the voltage between the center and outer
conductors of the A input only. The lock-in does
not force the shield of the A cable to ground,
rather it is internally connected to the lock-in's
ground via a resistor. The value of this resistor is
selected by the user. Float uses 10 kΩ and
Ground uses 10 Ω. This avoids ground loop
problems between the experiment and the lock-in
due to differing ground potentials. The lock-in lets
the shield 'quasi-float' in order to sense the
experiment ground. However, noise pickup on the
shield will appear as noise to the lock-in. This is
bad since the lock-in cannot reject this noise.
Common mode noise, which appears on both the
center and shield, is rejected by the 100 dB
CMRR of the lock-in input, but noise on only the
shield is not rejected at all.
Experiment
Signal
Source
Grounds may be at different potentials
convenient while the
SR810 Lock-In
A
+
-
R
Differential Voltage Connection (A-B)
The second method
differential mode. The lock-in 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.
When using two cables, it is important that both
cables travel the same path between the
experiment and the lock-in. Specifically, there
should not be a large loop area enclosed by the
two cables. Large loop areas are susceptible to
magnetic pickup.
Experiment
Signal
Source
Grounds may be at different potentials
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. Notice that the signal source is held near
ground potential in both illustrations above. If the
signal source floats at a nonzero potential, the
signal which appears on both the A and B inputs
will not be perfectly cancelled. The common mode
rejection ratio (CMRR) specifies the degree of
cancellation. For low frequencies, the CMRR of
100 dB indicates that the common mode signal is
canceled to 1 part in 10
100 dB, a 100 mV common mode signal behaves
like a 1 µV differential signal! This is especially
bad if the common mode signal is at the reference
frequency (this happens a lot due to ground
loops). The CMRR decreases by about 6
dB/octave (20 dB/decade) starting at around
1 kHz.
3-16
of connection
SR810 Lock-In
A
+
-
B
Loop
R
Area
5
. Even with a CMRR of
is the