Specifications

5
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 eliminat-
ing 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 conduc-
tive paths that carry ground currents.
If necessary, float the power supply
(don’t connect either terminal directly
to ground).
Minimize radiated pick-up (both elec-
tric and magnetic) by using twisted
shielded conductors for the output
and remote sense leads. To make sure
the shield doesn’t carry current, con-
nect 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 pre-
vent 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
Tip
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