Probe Considerations for Low Voltage Measurements such as Ripple

Probe Considerations for Low Voltage
Measurements such as Ripple
Application Note
This application note describes considerations for making
low voltage measurements using an oscilloscope and an
oscilloscope probe. Ripple measurements will be described as
an example of a low voltage application.
Application Note
Figure 1. 2X Probe (CH1) and 10X Probe (CH2) Lowest System Vertical Sensitivity.
The probes that ship with an
oscilloscope may not be the right
choice for low voltage measurements
Nearly all oscilloscopes ship with 10X attenuation passive
probes because this type of probe is the best choice for
making measurements across a broad range of applications.
To cover the widest range of applications, a general purpose
probe is usually rated from DC to 500 MHz and is generally
capable of measuring up to a few hundred volts. Users
making low voltage measurements will often fall into the trap
of using the 10X probe that came with their oscilloscope and
they end up with inaccurate results because a 10X passive
probe is not capable of making accurate measurements in the
low millivolt range.
Probe Considerations when making
Low Voltage Measurements
When making low voltage measurements, it is important to
consider oscilloscope sensitivity, probe attenuation, system
noise, probe grounding, probe input impedance, AC coupling,
probe offset, and probe bandwidth.
1. Use probes with a low attenuation factor to
maximize the oscilloscope’s vertical sensitivity
Vertical sensitivity indicates how much the oscilloscope’s
vertical amplifier can amplify a signal. On most Tektronix
oscilloscopes, the most sensitive vertical setting is 1 mV/
division when no probe is attached. As shown in Figure 1, the
measurement system’s smallest vertical scale factor will be 2
mV/division when a 2X probe is attached (Channel 1) is, and
the smallest vertical scale factor is 10 mV/division when a 10X
probe is attached (Channel 2).
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Figure 2. Probe (CH1) and 10X Probe (CH2) 10 mV Measurement.
Many Tektronix oscilloscopes have 10 vertical divisions. Using
a 10X probe with the system in the 10 mV/division setting, a
100 mV signal would completely fill the screen (10 mV/division
times 10 divisions). If 10 mV is again used as an example of
a low voltage measurement, this signal would only span 1
vertical division on the screen using a 10X attenuation probe
with the oscilloscope channel adjusted to its smallest vertical
scale of 10 mV/division. This example is shown as the blue
trace on Channel 2 in Figure 2. However, this same 10 mV
signal measured with a 2X probe will span 5 vertical divisions
since the vertical sensitivity of this channel can be adjusted to
2 mV/division. The 10 mV measurement with a 2X attenuation
probe is also shown in Figure 2 as the yellow trace on Channel 1.
The user should always set the volts/division so that the signal
nearly fills the screen. If not, the signal cannot be viewed with
greater detail and the scope’s digitizer is not fully utilized. In the
10 mV measurement example above, only 1/10th of the scope’s
digitizer is being used when a 10X attenuation probe is attached
since the signal only spanned 1 vertical division on the screen.
When the 2X attenuation probe is attached, the signal is able
to span 5 vertical divisions and half of the digitizer is now being
used. When more of the digitizer is used, signals are captured
with greater resolution.
Probe Considerations for Low Voltage Measurements such as Ripple
In order to use Equation 1, VIN and VNoise have to be
determined. If VIN is assigned a value of 10 mV as an example
of a low voltage measurement, the oscilloscope would be in
the 1 mV/division setting regardless of probe attenuation. An
example of an oscilloscope specification for Random Noise
is 150 uV + 8% of the volts/division setting, and in the 1 mV/
division setting, VNoise is 230 uV. SNR calculations for a 10X
probe and 2X probe using these values for VIN, VNoise and
probe attenuation are:
Figure 3. Input signal, probe attenuation, and random noise.
SNR calculation using a 10X probe
2. Use probes with a low attenuation factor to improve
the measurement system’s signal-to-noise ratio
A probe’s attenuation factor (i.e. 1X, 10X, 100X) is the amount
by which the probe reduces the amplitude of the oscilloscope’s
input signal. A 1X probe doesn’t reduce or attenuate the input
signal while a 10X probe reduces the input signal to 1/10th of the
signal’s amplitude at the scope input. As shown in Figure 3, the
input voltage arrives at the scope input divided by the probe’s
attenuation factor, shown as VIN divided by Attenuation.
Probe attenuation extends the measurement range of an
oscilloscope, allowing signals of greater amplitude to be
measured. However, when measuring low voltage signals, the
probe attenuates the signal and then the oscilloscope amplifies
it which causes a decrease in signal-to-noise ratio. The equation
for signal-to-noise ratio (SNR) is:
SRN =
VIN
(Attenuation) (VNoise )
VNoise is typically specified
as Random Noise in
oscilloscope data sheets
SRN =
10 mV
(10) (230 uV)
= 4.3:1
SNR calculation using a 2X Probe
SRN =
10 mV
(2) (230 uV)
= 21.7:1
Equation 2. SNR Calculations on a 10 mV Measurement using 10X and 2X Attenuation
Probes.
A 2X probe has a 21.7:1 signal-to-noise ratio for a 10 mV
measurement while a 10X probe has a 4.3:1 SNR. Clearly, a
lower attenuation probe increases the measurement system’s
ratio of signal-to-noise, which makes this probe a better
choice for low voltage measurements.
Equation 1. Calculation for Signal-to-Noise Ratio.
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Application Note
3. Be careful using long ground leads, especially near
the transformer and switching elements
A long ground lead is convenient because the user can make
one ground connection and probe many test points within
the range of the ground lead. However, any piece of wire has
distributed inductance, and the distributed inductance reacts
to AC signals by increasingly impeding AC current flow as
signal frequency increases. The ground lead’s inductance
interacts with the probe input capacitance to cause ringing
at a certain frequency. The ring frequency is described by the
following formula:
Figure 4. Chassis mount test jack.
f=
1
2
LC
Where:
V = the ring frequency
L = the inductance caused by
the probe’s grounding solution
C = the probe’s input capacitance
Equation 3. Ring Frequency Calculation.
This ringing is unavoidable, and may be seen as a sinusoid
of decaying amplitude. As the length of the ground lead
increases, the inductance increases and the measured signal
will ring at a lower frequency. The effects of ringing can be
reduced by limiting the length of the probe’s grounding or by
choosing a probe with lower input capacitance.
One simple solution to improve the ring frequency is to use a
shorter piece of grounding wire such as a short ground spring.
A picture of a probe with a short ground spring is shown
on the left side of Figure 4. With a short ground spring, the
inductance is reduced, decreasing the LC value and pushing
the inductive ringing out past the frequency range of interest.
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A grounding solution with the least amount of inductance
while obtaining a secure ground connection is a probe tip
chassis mount test jack (Tektronix part number 131-4210-00)
shown on the right side of Figure 4. The jack can be inserted
into the user’s test board and reduces the ground lead length
to almost zero.
The grounding wire can also act as an antenna or a loop,
causing capacitive and magnetic coupling effects. An
added benefit of reducing the ground lead length is reduced
exposure to radiated emissions near the transformer and
switching devices. If a longer ground lead is required, the
user should be careful not to place the grounding wire near a
transformer or switching device.
Probe Considerations for Low Voltage Measurements such as Ripple
4. Use probes with high input impedance
When a probe is inserted into a circuit, the probe will have
some effect on the circuit under test. A probe has resistive,
inductive and capacitive elements and one can imagine
that if a resistor, capacitor, or inductor was inserted at the
measurement point, it would change the behavior of the
circuit. Users should be aware of a probe’s input impedance
specifications to minimize the effects of probe loading.
The interaction of resistive, inductive and capacitive elements
produces total probe impedance that varies with signal
frequency. To minimize probe loading, the user should use the
shortest ground leads possible to minimize the inductance
and use a probe with low input capacitance. An active or
differential probe will offer the lowest input capacitance.
Another option may be a low input capacitance passive probe
such as the TPP0502, which is the only low attenuation, high
bandwidth, and high impedance passive probe in the industry
that is suited to make low voltage measurements on signals
with high frequency content. Again, this relationship is shown
in the ring frequency formula shown above in Equation 2 as a
relationship between frequency, inductance, and probe input
capacitance.
5. Use the oscilloscope’s AC coupling feature or adjust
the probe’s offset
One measurement challenge is measuring a low voltage AC
signal riding on top of a DC signal. There are a couple of
options to help users focus in on the AC portion of the signal.
When using an active probe, users should use the probe’s
offset control. Probe offset can be used to subtract out the DC
component of the signal in the probe amplifier. With the DC
portion of the signal removed, the user may now accurately
evaluate and measure the AC portion of the signal. On select
differential probes, Tektronix offers a feature called DC Reject.
DC Reject automatically generates an internal offset that
cancels the DC component of the signal.
When using a low attenuation passive probe to look at these
kinds of AC signals, users should use the AC coupling feature
on the oscilloscope to block the DC component and show
only the AC signal. For example, with AC coupling enabled on
the signal path of the oscilloscope and using the TPP0502,
engineers can evaluate the AC portion of the signal down to
2 mV of resolution. This method allows the user to separate
out the AC portion of the signal without having to add in the
DC offset. In some cases, this kind of measurement may not
be possible with an active probe due to the measurement
system’s limited offset range or the inability of the probe’s
amplifier to tolerate the large DC input.
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Application Note
of < 50 ns. Many users making “Power” measurements
would consider 6 MHz to be fast enough since 6 MHz is
more than enough bandwidth to make a measurement on a
design with switching frequencies below 1 MHz. Using the
P2220 probe as part of the measurement system, the ripple
characterized in Figure 5 appears very well behaved. However,
the measurement on Channel 1 using a low attenuation, high
bandwidth probe (TPP0502) yields a very different result. The
low bandwidth of the P2220 masks high frequency signal
content. The yellow trace in the highlighted area shows 50 mV
of ringing. By contrast, the signal captured with the P2220
only shows ~5 mV of ringing.
Figure 5. Low and High Bandwidth Passive Probes Capturing a Signal with Fast Edge
Speeds.
6. Use a probe with sufficient bandwidth
The rule-of-thumb for selecting a probe with sufficient
bandwidth is the probe should have five times the bandwidth
of the signal being measured. Bandwidth is a valid
specification when evaluating simple signals such as a sine
wave and characterizing what is occurring in the frequency
domain. However, most signals are complex and contain
many spectral components which may have frequency
values orders of magnitude higher than the fundamental
frequency. For signals that change quickly over time and
have a fast slew rate (dv/dt), the measurement system must
be capable of capturing these events over time and must be
able to accurately characterize what is occurring in the time
domain. The specification that determines how effective the
measurement system is over time is the rise time specification.
Rise time is an important specification when considering the
measurement system’s capability of evaluating rising and
falling edges and for capturing higher order harmonics. Many
times, users conclude that since their signal is “not that fast”,
they select a probe with insufficient bandwidth or better
stated, they select a probe with insufficient rise time capability.
Consider the ripple measurements shown in Figure 5. The blue
trace shown on Channel 2 was captured using a P2220 probe
in the 1X attenuation setting. In the 1X attenuation setting, the
P2220 has 6 MHz bandwidth and has a rise time specification
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What are the right probes for making
low voltage measurements?
The best probes for making low voltage measurements are
active or differential probes. One example is the Tektronix
TDP1500 differential probe which has selectable 1X and
10X attenuation ranges. In the 1X setting, this probe
doesn’t reduce or attenuate the signal and will produce a
measurement with greater SNR and greater resolution. One
of the features that make a differential probe the best choice
for a low voltage measurement such as ripple is its common
mode rejection capability. Common mode rejection allows the
probe to reject signals common to both probe inputs such as
the coupling that can occur from a transformer or switching
nodes. Active and differential probes also typically have higher
bandwidth and lower probe loading effects.
A lower cost alternative with good performance is the
Tektronix TPP0502. The TPP0502 shares the advantages
of both passive and active probes, offering ruggedness,
performance, and lower cost. Along with its low, 2X
attenuation range, the TPP0502 offers high bandwidth (500
MHz), large dynamic range (300 V CAT II), and high input
impedance at the probe tip with banner specifications of 2 MΩ
and 12.7 pF. With its 500 MHz bandwidth, the TPP0502 offers
significantly better performance than other low attenuation
passive probes in the industry which offer a maximum of
25MHz of bandwidth. When a probe has limited bandwidth, it
can cause the user to miss frequency content which may be
affecting in the signal under test.
Probe Considerations for Low Voltage Measurements such as Ripple
Figure 6. 3.3 V supply measured with a 2X Probe (blue) and a 10x probe (yellow).
Figure 7. Probing a 3.3 V Power Supply with a 2X Attenuation Passive Probe.
Ripple Measurement Case Study
A design engineer was struggling with power supply noise.
He needed to probe a 3.3 V supply, but the 10X probe
he was using didn’t provide him with enough sensitivity to
see the small ripple voltage and he couldn’t trigger on the
periodic noise present in the waveform. Separating noise from
ripple voltage can be a significant problem, and this kind of
measurement requires a probe with low attenuation. Consider
the waveforms in Figure 6. The yellow trace is a 10X probe
adjusted to the lowest vertical setting of 10 mV per division
and a 2X probe (TPP0502) is the blue waveform. The 2X
probe can be adjusted down to its lowest vertical setting of
2 mV per division. The output of the power supply produces
a signal with 3 mV of ripple, and it is clear why a probe
with 10X attenuation is not effective for making low voltage
measurements.
The design engineer understood the benefits of low
attenuation probes for making low voltage measurements.
The engineer made this comment regarding the usability of
the TPP0502 - “It was a very simple matter to AC couple the
signal and find the 25kHz ripple that was getting through the
front end. The ease of use of this probe is fantastic- it’s really
just point and shoot. Doing the same measurement with an
active probe means having to figure out the offset you need
and dial it in. Also, thanks to the 300V CAT II rating, I didn't
have to worry about accidentally touching one of the adjacent
high voltage lines in the power supply. Since it has plenty of
bandwidth, I was also able to see some of the other spikes in
the power supply that I could be doing a better job of filtering
out!” Figure 7 is a plot of the TPP0502 probing a 3.3V power
supply node with the scope’s vertical sensitivity set to 2mV/
div, the input channel set to AC coupling, and the scope’s
acquisition mode set to single shot. The white reference trace
is the power supply that has the noise leaking into the vertical
channel, and the yellow trace is the same signal that is better
behaved.
Conclusion
When making low voltage measurements, it is very important
to evaluate probes by looking at their attenuation and input
impedance specifications. While active or differential probes
like the TAP1500 and TDP1500 are the most effective choices
for measuring low voltages, the TPP0502 from Tektronix is an
economical, general purpose low attenuation passive probe
that is also very capable of providing accurate low voltage
measurements.
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