Technical Insights > Benchtop

Power Integrity: Quality Defines Performance in Electronics

2021-02-09  |  10 min read 

The functional reliability of a product is directly proportional to the quality of the DC power inside. Stable DC supplies should not cause issues. Unstable DC supplies can cause unreliable performance. The density of integrated chips (ICs) in our products is still increasing to accommodate more intelligent sensors, increase the speed of more data, and support more features. A larger number of smaller components packed onto each board makes your product more susceptible to the effects of poor power supply and management. To minimize problems, your design must convert and deliver DC power as effectively as possible (e.g., from converters to gates on the ICs). And if you want your design to have high power integrity, testing and verifying power integrity is important.

Tests Required to Validate Power Integrity

Evaluation usually consists of these four steps:

1. Analyze the output of your DC/DC converters without the rest of the circuit turned on.

•    This is to test the supply’s stability, looking for drift and PARD (Periodic and Random Disturbances)

2. Turn on your system and stress the supply under various operating conditions.

•   For example, test static and dynamic load to check the response and high-frequency switching while keeping an eye out for transients and noise.

3. If your system has different power saving modes, you’ll evaluate your programmable power rails.

•   You want to ensure your supplies are reaching their intended level with the appropriate latency.

4. Lastly, run some (or all) of these tests again in a temperature chamber or accelerated life tester.

•   It is important to check operation in extreme environmental conditions and gain insight into how your device will perform over time.

Challenges in Making Power Integrity Measurements

For all the tests described above, you have a specific tolerance band. If AC signals riding on your DC signal deviate too much, you have poor power integrity and your design is flawed. There are two major challenges to measuring your power integrity: noise and offset.


Noise


The collective noise from your oscilloscope, probe, and the connection to the device under test (DUT) are mixed in with your signal when you measure it. The result is that you don’t see an exact version of your signal on the oscilloscope screen. The best option in getting accurate readings is to use a high-quality measurement system:


•    Choose an oscilloscope with low noise
•    Choose a probe with low noise and 1:1 attenuation
•    Connect to your DUT using the shortest lead possible, with minimal to no probe-tip accessories


Following these guidelines ensures you won’t mistake measurement system noise for power rail noise.Offset
Viewing your AC swing can be difficult when your DC signal is large. To see the full signal on the screen, you have to zoom out quite far, but then you aren’t looking closely at the AC details. So, what do you do? Use a probe with support for power rail voltages. This is a probe with enough offset to be able to center the signal on screen without blocking DC so you can zoom in on the details of your waveform.


Probe offset is better than using a DC block because:

1.    Blocking capacitors not only block DC, but they also block or filter low-frequency AC. This inhibits the ability to see any drift, droop, sag, and other changes to the DC value of the power rail. These attributes are often critical observations when your FPGAs and microprocessors turn on and off.
 

USB device voltage measured with a DC block
Figure 1.  USB device voltage measured with a DC block

Probe offset passes all the AC content to the oscilloscope unfiltered.

Same USB device but measured with probe offset
Figure 2.  The same USB device as in Figure 1, but measured with probe offset

In Figure 1, you can see the DC block shows what looks like a stable DC supply. In reality, the supply has some issues that become visible using the power rail probe in Figure 2. The issues can’t be seen with the DC block because it filters out the low-frequency drift in the supply.

2. When using a DC block, the capacitor can discharge into your oscilloscope and even blow out the front end. This is because the power rail you are measuring may exceed the input voltage of the oscilloscope, and the capacitor is being charged with that voltage. You may think you are protecting the oscilloscope from the voltage of your device, but if the capacitor discharges, all that energy could get sent into the front-end of your oscilloscope; a costly repair.
 
3. DC blocks can make documenting results tedious. A DC block inhibits all DC information from arriving at your oscilloscope. As a result, the oscilloscope will show the waveform centered at zero volts. Therefore, you need to use a digital multimeter (DMM) to find the nominal value of the supply and then manually type this information into any saved data or screenshots. Using a probe with offset means the oscilloscope knows the DC offset and can display things correctly, which makes record keeping much easier. The DC offset is considered in any automated measurements or applications.

Additional Challenges – Loading and Bandwidth

Probe loading can cause your power supply to behave differently than it does without the probe connected or can cause measurement errors like sag. So, you’ll also want to use a probe with very low loading.
You also want to choose a probe with high bandwidth. Devices running at increased speeds can introduce crosstalk on boards with small dimensions and lanes (traces) placed close together. With an increased risk of crosstalk, you’ll need to see transient signals, which requires high bandwidth. Having more bandwidth is also helpful for viewing high-frequency supply noise, which can cause electromagnetic interference.


The Best Probe for Power Rail Measurements

Here is a summary for overcoming power integrity measurement challenges; use a probe with:

1. Low noise
2. Support for popular rail voltages
3. Low loading
4. High bandwidth

If you need a specific product suggestion, use the Keysight N7020A or N7024A power rail probes.  They meet the criteria suggested above and are summarized below:
 

1. Low noise:

•    The N7020A adds only 10% of the oscilloscope noise
•    The N7024A adds only 30% of the oscilloscope noise

2. Support for popular rail voltages:

•    The N7020A has an offset range of ±24V
•    The N7024A has an offset range of ±15.25V

3. Low loading:

•    The N7020A has an offset range of ±24V
•    The N7024A has an offset range of ±15.25V

4. High bandwidth:

•    The N7020A has 2 GHz of bandwidth
•    The N7024A has 6 GHz of bandwidth

Both probes work with Keysight Infiniium oscilloscopes, which have amazing signal integrity, low noise, and plenty of bandwidth. Additionally, they are compatible with special probing tips that help probe common surface mount capacitors packages.

Attribute N7020A N7024A
Probe bandwidth (-3dB) 2 GHz 6 GHz
Attenuation ratio 1.1:1 1.3:1
Offset range ± 24V ±15.25V
Input impedance at DC 50kΩ +/-2% 50kΩ +/-2%
Probe noise 0.1 * scope noise 0.3 * scope noise
Active signal range ± 850mV about offset voltage ± 600mV about offset voltage
Probe type Single-ended Single-ended
Included accessories N7021A - Coaxial pigtail probe head (qty 3): 8” N7032A 4 GHz browser for 0603 and 0805 packages (inch code)
Included accessories N7022A - Main cable: 48” N7033A 5 GHz browser for 0201 and 0402 packages (inch code)
Included accessories N7023A – 350 MHz browser: 45”  
Not included, order separately   1250-4403 Rotating SMA adapter
Output impedance 50Ω 50Ω
Extended temperature range N7021A main cable, N7022A pigtail probe head: -40° to + 85° C