ATX/ATX12V Power Supply Design Guide
Version 1.1
Page 15
3.2.6. Output Ripple/Noise
The output ripple/noise requirements listed in Table 11 should be met throughout the load
ranges specified in Section 3.2.3 and under all input voltage conditions as specified in
Section 3.1.
Ripple and noise are defined as periodic or random signals over a frequency band of 10 Hz
to 20 MHz. Measurements shall be made with an oscilloscope with 20 MHz bandwidth.
Outputs should be bypassed at the connector with a 0.1 µF ceramic disk capacitor and a
10 µF electrolytic capacitor to simulate system loading. See Figure 1.
Table 11. DC Output Noise/Ripple
Output
Max. Ripple & Noise
(mV
pp
)
+12 VDC
120
+5 VDC
50
+3.3 VDC
50
-5 VDC
100
-12 VDC
120
+5 VSB
50
Scope
Load
V out
V return
Power Supply
AC Hot
AC Neutral
AC Ground
Scope Note:
Use Tektronix TDS460 Oscilloscope or
equivalent and a P6046 probe or equivalent.
Filter Note:
0.1uf - Kemet, C1206C104K5RAC or equivalent
10uf - United Chemi-con, 293D106X0025D2T or
equivalent
General Notes:
1. Load the output with its minimum load
current.
2. Connect the probes as shown.
3. Repeat the measurement with maximum
load on the output.
Load must be
isolated from the
ground of the
power supply.
0.1uf
10uf
Figure 1. Differential Noise Test Setup
