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.