Stanford Research Systems SR865A, Operation Manual

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56 Basics Chapter 2
SR865A DSP Lock-in Amplifier
Many of these noise sources can be minimized with good laboratory practice and
experiment design. There are several ways in which noise sources are coupled into the
signal path.
Capacitive coupling
An ac voltage from a nearby piece of apparatus can couple to a detector via a stray
capacitance. Although C
stray
may be very small, the coupled noise may still be larger than
a weak experimental signal. This is especially troublesome if the coupled noise is
synchronous (at the reference frequency).
We can estimate the noise current caused by a stray capacitance by:
noise stray stray
V C
V
C
Z
dt
d
i
where ω is 2π times the noise frequency, V
noise
is the noise amplitude, and C
stray
is the
stray capacitance.
For example, if the noise source is a power circuit, then f = 60 Hz and V
noise
= 120 V.
C
stray
can be estimated using a parallel plate equivalent capacitor. If the capacitance is
roughly an area of 1 cm
2
separated by 10 cm, then C
stray
is 0.009 pF. The resulting noise
current will be 400 pA (at 60 Hz). This small noise current can be thousands of times
larger than the signal current. If the noise source is at a higher frequency, the coupled
noise will be even greater.
If the noise source is at the reference frequency, then the problem is much worse. The
lock-in rejects noise at other frequencies, but pick-up at the reference frequency appears
as signal!
Cures for capacitive noise coupling include:
1) Removing or turning off the noise source.
2) Keeping the noise source far from the experiment (reducing C
stray
). Do not bring
the signal cables close to the noise source.
3) Designing the experiment to measure voltages with low impedance (noise current
generates very little voltage).
4) Installing capacitive shielding by placing both the experiment and detector in a
grounded metal box.