Specifications

4
Eliminate noise from
low-level measurements
Noise in low-level measurements
can come from a number of different
sources, and it’s easier to eliminate
noise than to filter it. Check these
noise sources:
1. Power supply
Starting with a low-noise supply is
naturally a great way to keep noise
out of your measurements. Linear
power supplies have lower common-
mode noise currents and generally
operate at low frequency. However,
you can use switch-mode supplies
successfully if their specifications
include a low common-mode current.
As a rule of thumb, common-mode
current over 20-30 mA is likely to
cause trouble. Keep reading for hints
on how to minimize the problem.
2. DUT to power supply
connections
Minimize conducted noise by
eliminating ground loops. Ideally,
there should be only one connection
to ground. In rack systems, where
multiple ground points are inevitable,
separate the dc distribution path from
other conductive paths that carry
ground currents. If necessary, float
the power supply (don’t connect
either terminal directly to ground).
Minimize radiated pick-up (both
electric and magnetic) by using
twisted shielded conductors for the
output and remote sense leads. To
make sure the shield doesn’t carry
current, connect the shield to ground
at one end only, preferably the single-
point ground on the supply (Figure 1).
Minimize the power supply’s common-
mode noise current by equalizing the
impedance to ground from the plus
and minus output terminals. Also
equalize the DUT’s impedance to
ground from the plus and minus
input terminals. Magnetic coupling
or capacitive leakage provide a return
path for noisy ground loop current at
higher frequencies. To balance the
DUT’s impedance to ground for your
test frequencies, use a common-mode
choke in series with the output leads
and a shunt capacitor from each lead
to ground.
3. Current variations to the DUT
Rapid changes in the DUT’s current
demand cause voltage spikes. To
prevent this, add a bypass capacitor
close to the load. The capacitor should
have a low impedance at the highest
testing frequencies. Avoid imbalances
in load lead inductance; direct
connections to the DUT, such as
twisted shielded pair, are your best
bet.
Figure 1: Minimizing radiated pick-up with twisted shield leads for both output and
remote sense leads
Power
supply
Load
Shield
Load
C
Shield
+
–
+S
–S
Tip
3