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52
Basics
Differential Voltage Connection (A−B)
The second method of connection is the 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. The shields of A and B are
connected together and can set to Float or Ground as above.
When using two cables, it is important that both cables travel the same path between the
experiment and the lock-in. In particular, 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 on the center of 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
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 often happens due to
SR865A DSP Lock-in Amplifier
Chapter 2
5
. Even with a CMRR of 100 dB,
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