Datasheet

Measuring Low Level Ripple Voltage
using the Model DMM7510
7½-Digit Graphical Sampling Multimeter
Introduction
Power conversion is a process that exists in all electronic
devices. The term “power supply” is loosely interpreted to
describe this process, which conditions the power from the
primary source to have specific characteristics required by the
load. Digital multimeters (DMMs) is a very common instrument
used to evaluate many power supply specifications. However,
traditional DMMs often lack the capabilities to measure dynamic
behaviors, especially those found in the class of switch mode
power supplies (SMPSs). An oscilloscope is typically needed to
examine the supply’s ripple voltage, switching voltage, power-up
glitches, etc. In many cases, an oscilloscope might not offer the
resolution needed to correctly characterize the very low level
ripple noise. Keithley’s Model DMM7510 7½-Digit Graphical
Sampling Multimeter can sample voltages down to 1µV or current
down to 100pA sensitivity at 1µs intervals. In other words, the
Model DMM7510 allows you to digitize a variety of dynamic low
level voltage and current waveforms with 18-bit accuracy at a
1Mega-sample per second sampling rate. Combining with the
instrument’s multi-touch touchscreen interface technology, you
can easily capture, view, and interact with both steady-state and
dynamic signals from power supplies.
This application note illustrates how to capture, view and
characterize the dynamic ripple voltage on a DC-to-DC buck
converter on the Model DMM7510’s front panel user interface.
Understanding the basics of a buck converter
A buck converter is a highly efficient switch mode DC-to-DC
voltage step down converter. It stores energy in the form of a
magnetic field on an inductor. In the “ON” state, the switch is
closed and the input voltage charges up the inductor. In the
“OFF” state, the switch is open and the inductor discharges the
stored energy as current flows through the load.
For the purposes of demonstration, this application note
uses Texas Instruments LM25088 DC-to-DC buck converter
evaluation board (EVM). It produces a steady 5V output from a
5.5V–36V on the input. Modifications were made to the LM25088
EVM to realize a 50kHz switching frequency that is well below
the 600kHz analog bandwidth of the instrument at the 10V
range. Refer to the instrument specifications from Keithley’s
website (www.keithley.com) for additional analog bandwidth
requirements for different voltage and current ranges.
Ripple Noise on the Output Voltage
A small ripple voltage will very likely exist on the output of a
buck converter when it is on. This phenomenon is caused by the
switching frequency, output capacitance, inductor, load, and any
current-limiting features. Because it is an undesirable effect of
the design, designers will examine and minimize this noise.
Making Connections from the Buck
Converter to the Model DMM7510
The buck converter can be easily connected to the Model
DMM7510 using a standard pair of test leads as shown in
Figure 2 . Properly shielded test wires are recommended when
measuring low level signals. To conduct the test, the buck
converter is powered by a 12V DC linear power supply. A
linear power supply provides a much lower noise output than a
switching power supply, ensuring that the waveform produced
reflects the noise from the DUT rather than the noise from the
input. The output of the buck converter is connected to a 3Ω
resistive load.
DC Load
Figure 2. Ripple voltage test connections.
Setting up the Ripple Voltage Measurement
from the Front Panel
The ripple voltage can be easily captured by the Model
DMM7510. With only a handful of key strokes to set up the
Application Note
Se ries
DC
ON
DC
Load
Load
OFF
Figure 1. DC-DC buck converter operation.