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Measuring LDO Power Supply Rejection Ratio for a Novice

As somewhat of a continuation of my last blog topic I am once again returning to measurement of an LDO parameter as I look at the power supply rejection ratio (PSRR) of this device. As I have been transitioning into this new role I have found that products going into space require much more stringent specifications and review than typical commercial products. Hence, I have found myself evaluating in depth what I always considered a pretty simple device, an LDO. As it turns out I have found out that I was quite the novice with this type of device, hence the title of this blog and my last blog Measuring LDO Noise Spectral Density for a Novice. There are many additional things to think about when working with space products that aren’t necessary to consider for commercial products. I mentioned last time that one of these things is the harsh radiation environments that are present in space.

For a little more insight into that topic you can check out two-part blog series from one of my colleagues at Jupiter: The IC Danger Zone, Part 1 as well as Jupiter: The IC Danger Zone, Part 2. Another example is more stringent and specific test requirements which I am providing some insight into in these latest blogs. I am showing the procedures to test these parameters as I have been looking at these parameters over temperature and process as a part of the evaluation for their space worthiness. Recall that I mentioned in the last blog that systems designed to go into space must be essentially failure proof. It just is not feasible to fly up to a satellite in orbit and replace a defective or damaged component. This translates back to the system components which must be tolerant of radiation in space, operate over temperature extremes, and are generally in hermetically sealed packages to name just a few requirements.

I have been making these measurements in the lab mainly because of the special requirements for space products, but also because these types of measurements do not lend themselves well to an ATE (automated test equipment) environment. These types of tests are for all intents and purposes not suitable for ATE (at least not efficiently). Similar to the Noise Spectral Density (NSD) measurement performing a PSRR measurement on an LDO poses some challenges because the measurement frequencies are very close to DC. Once again, I researched the topic and talked to several folks within Analog Devices before arriving at a proper test procedure.

As I did in my previous blog, I offer this information to you as the reader so you can avoid the lengthy time that I spent attempting to figure out what turned out to be a relatively simple measurement (at least once you know what you are doing!). As you may have heard before it generally is all about having the right tool. In this case I was able to get the right tool on loan from the great folks at Keysight, combine that with a little expertise from same great sales and applications folks at Keysight on how to set up and use this tool, and then I was on my way to making measurements.

In order to perform a PSRR measurement on an LDO there are two pieces of equipment that are necessary. The first is a DC power supply to power the LDO input voltage. Once again, a Keysight (Agilent) E3631A DC power supply such as the one pictured below is used. The second and most important piece of equipment is the Keysight E5061B Network Analyzer. The key here is to have the low frequency input option which is an option that gives a frequency response down to 5 Hz. It is also key that the T, R, and LF OUT ports are used which can accept a DC input.

In addition we need a special device called a line injector that will allow an AC signal to be combined onto the DC input voltage. At the suggestion of the folks at Keysight I was able to locate a nice little device from Picotest in order to accomplish this task. I used the model J2120 pictured below to inject the AC signal onto the input of the LDO.

The DC power supply should be connected to the DC port of the Picotest J2120 line injector. The LF port of the E5061B network analyzer connects to the OSC port of the line injector. The output of the line injector then connects to the input terminal of the LDO. The T and R ports of the E5061B may be connected up in two different ways depending on the desired polarity of the PSRR measurement. With the T port connected to the input of the LDO and the R port connected to the output of the LDO the PSRR measurement results in a negative polarity. Reversing the connections results in a positive polarity in the PSRR measurement.

For this particular case the LDO under test requires an input voltage of 5.5V and has outputs of varying voltages from 3.0V to 5.0V. Banana to clip lead cables are used to connect the E3631A DC power supply to the VDD voltage input of the LDO evaluation board. The LDO evaluation board has SMA connectors as well as header pins on all the voltage outputs which allow for easy connection to the E5061B with either BNC to SMA cables or with BNC to clip leads (as shown below). In the image below notice the input voltage supplied from the E3631A is quite a bit higher than 5.5V. This is to overcome the loss through the line injector. It is important to verify the correct input voltage is present at the input to the LDO. Also notice that the T port is connected to the LDO input while the R port is connected to the LDO output yielding a positive polarity in the PSRR measurement.

Power Supply Rejection Ratio Measurement Setup for an LDO

Power Supply Rejection Ratio Measurement Setup for an LDO

Once all the connections are made the Keysight E5061B must be set up to properly measure the PSRR of the LDO. There are a few simple steps to set up the E5061B to make this measurement. Below is a list of settings that I used, but some of these could be different depending on the specific application requirements. These settings should at least be a good place to start.

  1. Connect DC power supply to the DC input of the Picotest J2120A line injector.
  2. Connect LF Out port of the Keysight E5061B to the OSC input of the Picotest J2120A line injector.
  3. Connect the OUT port of the Picotest J2120A line injector to the input of the LDO evaluation board.
  4. Connect the R port of the Keysight E5061B to the output of the LDO evaluation board.
  5. Connect the T port of the Keysight E5061B to the input port of the LDO evaluation board.
    1. Note this will result in a negative PSRR reading. To achieve a positive PSRR reading simply reverse the connections of the R and T ports to the LDO evaluation board.
  6. On the Keysight E5061B network analyzer configure the following settings:
    1. Press Meas
      1. Select Meas Port = Gain/Phase
      2. Select Gain/Phase Setup
        1. T input and R input Z = 1Mohm (default setting)
        2. T att and R att = 20 dB (default setting)
    2. Press Avg
      1. Select IF Bandwidth = 5 Hz
      2. Select IFBW Auto = On
      3. Select IFBW Auto Limit = 100 Hz
    3. Press Sweep Setup
      1. Select Power = -20 dBm (or appropriately level so device is not overdriven)
      2. Select CW Freq = 100 kHz
      3. Select Sweep Time = Auto
      4. Select Points = 200 (or greater if needed but more points slow sweep time)
  7. Next the Keysight E5061B must be calibrated.
    1. Connect the R and T ports as well as the OUT port of the Picotest J2120A line injector to the input of the LDO evaluation board.
    2. Press Cal
      1. Select Thru
        1. Select Thru Response
          1. When sweep is done select Done
    3. Reconnect the R port to the output of the LDO evaluation board.
  8. Collect data for the PSRR performance of the LDO.
    1. The screen image can be saved if desired from the System menu (Press System key).

    The trace data can be saved form the Save menu (Press Save/Recall key). As an example of the PSRR measurement, there is a typical PSRR plot included below that shows the negative polarity since the R port is connected to the LDO input and the T port is connected to the LDO output in this case.

    I hope you have had as much fun reading about this adventure as I have had on the journey. It has been quite a different undertaking making these measurements on an LDO. I was faced with some interesting challenges along the way but thanks to some great help from folks at Analog Devices as well as some key assistance from Keysight (pardon the pun) I was able to successfully make these measurements. Stay tuned as we continue to look at measuring more parameters of LDOs.

2 comments on “Measuring LDO Power Supply Rejection Ratio for a Novice

  1. hansaresta
    October 8, 2016

    Nice read. Some other important points to consider in measuring PSRR: 1.a. Avoid long wires in your set-up since these will add unwanted inductance that may affect your result. 1.b. Probe Vin and Vout (T and R) using high impedance probes and as close as possible to the DUT pin (again to minimize inductance effect) 2. Remove input capacitor/s of LDO before measurement. It could affect your line injector to go unstable at higher frequencies. Note that the input capacitor (and input impedance), depending on its value, also acts as a low pass filter to your AC signal that may introduce a lower cut-off frequency and may lead to an earlier 3dB roll-off of your PSRR (eg. at around 100kHz or less, PSRR may start to roll-off because the filter starts attenuating your input signal.) 3. and basic considerations like: 3.a. Vdc + Vac must be less than the absolute maximum supply of your LDO 3.b. Vdc – Vac must be greater than the UVLO of your LDO BTW: The usable bandwidth of J2120A Line injector is 10Hz~10MHz (set-up requires 5Hz~30MHz). Consider other methods if data outside 10Hz~10MHz is a crucial part of your measurement.

  2. jonharris0
    October 10, 2016

    Hi hansaresta, some very good points. 

    1a. I found that the BNC to alligator clips worked equally as well as the high impedance probes in this case. 

    1b. Indeed, you want to probe as close as you can to the input pins.

    2. Input capacitors must be removed in order to get an accurate measurement of the LDO, that is definitely an important point.  The output capacitors need to be left in place as often times these affect device stability.

    3a. Yes, the input signal needs to be small.  Notice the input amplitude setting is pretty small and may need to be lowered depending on the device saturation.

    3b. Indeed the specified BW is up to 10MHz, but the measurement seemed to work pretty well and I was able to calibrate it up to the 30MHz limit of the T/R ports on the E5061B.

    Thanks for your great comments!  You make some excellent points! Thanks for reading and thanks for taking the time to write in such a detailed and thorough response!

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