Specifications

Noise in low-level measurements
can come from a number of differ-
ent sources, and it’s easier to elimi-
nate 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 com-
mon-mode noise currents, and
generally operate at low frequency.
However, you can use switch-
mode supplies successfully if their
specifications include a low com-
mon-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 elim-
inating ground loops. Ideally, there
should be only one connection to
ground. In rack systems, where
multiple ground points are
inevitable, separate the dc distribu-
tion 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 sup-
ply (Figure 1).
Minimize the power supply’s com-
mon-mode noise current by equal-
izing 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 cur-
rent demand cause voltage spikes.
To prevent this, add a bypass
capacitor close to the load. The
capacitor should have a low imped-
ance at the highest testing frequen-
cies. Avoid imbalances in load lead
inductance; direct connections to
the DUT, such as twisted shielded
pair, are your best bet.
Eliminate Noise from
Low-Level Measurements
Tip
4
C
S
S
Power
Supply
Shield
Shield
Load
Figure 1: Minimizing radiated pick-up with twisted shield leads for
both output and remote sense leads.
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