In my last blog, A Quick Overview of Radiation Effects – Single Event Effects, we took a look at typical single event effects (SEE) to round out the discussion providing an overview on radiation effects. Now that we have a good idea of both the cumulative and single event effects we can begin looking at these effects with a high speed ADC. To begin we will start by looking at TID effects.
Recall from my blog back in December, A Quick Overview of Radiation Effects, that there are generally two types of TID effects that are observed, LDR (Low Dose Rate) and HDR (High Dose Rate). Due to the time involved to test for LDR effects it is generally not performed prior to product release since it could take several weeks to possibly months to complete. It is not a requirement to perform LDR testing so it is more common to perform only the HDR testing prior to product release. The testing could be accelerated with increased temperature but still takes much longer than high dose rate testing.
Even though it is not actually required to complete LDR testing for TID, Analog Devices in several cases performs this test after product release to provide further radiation performance information to customers. For this blog series looking at cumulative effects on a high speed ADC we will look at HDR testing for TID effects since the LDR data is not yet available. Specifically, we will look at the recently released AD9246S 14-bit 125 MSPS high speed ADC.
To analyze the HDR performance of a device such as the AD9246S the device is tested on an ATE (automated test equipment) solution prior to exposure to HDR radiation and then tested again on the ATE after the radiation exposure. The goal is to check for any appreciable shifts in device performance. In this case,the device was irradiated at VPT RAD out to 100 kRads total ionizing dose. When TID tests are performed, Analog Devices produces a test report detailing the conditions of the test and providing a summary of the performance pre- and post-irradiation. The test conditions are summarized on the front page of the report as can be seen below.
AD9246S Radiation Test Report for HDR TID
For the case of the AD9246S the dose rate during the test was 129 Rad/s out to the total ionizing dose of 100 kRads using Cobalt60 under test conditions specified by TM1019 Condition A. The test was performed at VPT RAD in July of 2016. One thing you may notice about this and the recent product release date is the delta in time. It is prudent to perform the radiation testing of a given device early in the product development process to better understand how the device will perform and to make decisions on any radiation performance improvements that may need to be made. If a given product has poor TID performance it may not be a good candidate for space applications unless appropriate radiation performance enhancements can be made (within time and budget constraints of course!).
There are two ways that data is present in the report; data is presented tabulated as well as in performance graphs. The report is several pages in length (110 pages to be exact), but we’ll only take a quick look at a few of the key parameters that are generally observed with a high speed ADC. For the full report you can go here: AD9246S TID Report. Notice in the table below that SNR, SINAD, ENOB, 2nd harmonic, 3rd harmonic, and SFDR are all presented. There were a total of five units, four irradiated units and one control unit, tested with results of the ATE test pre- and post-radiation exposure presented.
The ATE limits are given in the last row and the statistics are shown based on the devices tested (minimum, maximum, mean, standard deviation, mean + 3 sigma, and mean – 3 sigma). As you can see the AD9246S performs really well in these areas. All the performance parameters shown here easily meet the data sheet limits even after the 100 kRads exposure from the TID test. I’d like to note that the 2nd harmonic shows a mean – 3 sigma value less than the limit. This is due to the one unit where the 2nd harmonic was much less than the typical value which skews the sigma value. This is not an issue since the measured value was well below the limit, it simply skewed the sigma value slightly because of the large delta.
AD9246S Tabulated Data for AC Performance Metrics
There are many other tests that were performed on the ADC for TID testing in addition to what I have shown here. The device showed no significant shift in performance and is specified as a 100 kRADs TID product. In addition to the tabulated data there are many different performance plots shown as well that give the pre- and post-radiation average for all the parameters tested in the ATE solution. I have once again selected a few of the common parameters. I took several of the plots and combined them into one image so that we could take a look at the same parameters that are shown in the tabulated results given here.
AD9246S Performance Plots for AC Performance Metrics
Providing the performance plots provides a visual representation of the performance pre- and post-radiation exposure. The average plot gives an indication of how the average performance of the ADC shifts after the 100 kRads exposure during the TID testing. As can be seen for each of these parameters shown, the shift is very small. Similarly the mean + 3 sigma and the mean – 3 sigma shifts after the 100 kRads exposure are also shown to be very small. Notice once again that the very low value for the 2nd harmonic measured on two of the units skews the sigma such that the mean – 3 sigma is beyond the limit. Once again there is no concern here since the skew in sigma is from measurements significantly below the limit which is a move in a positive direction for performance.
There are specific methods related to the test procedures that I will not cover here for TID testing. The basics are that the device is tested on the ATE prior to the radiation exposure and then, within a specific amount of time and under a specific set of conditions, the device is tested on the ATE after the radiation exposure. All the tests performed on the ATE are recorded and presented in a report as presented here. This report helps potential users determine if the ADC is suitable in their space application.
Next up we will continue looking at radiation effects with high speed ADCs as we take a look at the various single event effects (SEEs). In terms of difficulty of test, checking the TID performance of a device is much less complex to perform than checking the SEE performance. The main reason this is so is because test evaluation is not performed during the radiation exposure for TID. When testing for SEE effects test evaluation is performed during radiation exposure. If you would like to explore more about TID before we move on to SEE more information can be found at NASA’s page which has more information as well as several links regarding TID at Radiation effects & analysis; TID effects. Stay tuned for my next blog where we will look at what SEEs are observed and how they are tested on a high speed ADC.