In Analog Devices Design Tools: ADIsimRF Part 1 of this series I introduced the ADIsimRF tool which again, can be downloaded for free from the Analog Devices website after filling out a short software request form here: ADIsimRF. Before looking into the signal chain portion of the tool we began by looking at the calculator tool available in ADIsimRF. This particular tool has many different calculation possibilities which are very nice to have on the hand when either in the lab collecting data or when working on a report in your office. These are great to double check expectations or to set expectations when about to take data or when analyzing data that previously been collected. Once again, the calculator can be accessed from the main page of ADIsimRF by selecting Tools from the menu bar.
This opens the ADIsimRF Calculator window that allows the user to select from many different useful calculators. Last time we took a look at the Vrms, Vpp, dBV, dBm, mW selection and did a short example using a typical ADC usage case. For the sake of time we won’t go through every calculator, but let’s take a look at a few more to get a feel for how helpful this tool can be. First, let’s look at the Decimal to Binary and Hex calculator. This is another handy tool for an ADC guy such as myself. This is a nice tool to convert between the different number formats quickly. In this example I’ve taken the positive full scale value of a fourteen bit converter in two’s complement format which in hexadecimal is 0x1FFF, in binary is 0b01111111111111, or in decimal is 8191.
Another important parameter to consider when dealing with an ADC is the noise spectral density. The NF and NSD in 50 ohms calculator tool will convert between the Noise Figure (NF) and Noise Spectral Density (NSD). It is important to remember though that the tool, being an RF tool, does this calculation in the 50 ohm domain which is the standard impedance in RF applications. When dealing with an ADC, the input impedance is not always set to 50 ohms. In many cases it can be 100 ohms, 200 ohms, or even as much as 400 ohms. If the impedance of the ADC input network is set to a value other than 50 ohms, then the calculator may not be used. However, the ADIsimRF tool can handle this with no problem when doing the signal chain analysis (which we will explore later in this series of blog posts). Please pay attention to the units as well. In RF terms it is more common to look at NSD in dBm/Hz whereas in ADC terms it is more common to look at NSD in dBFS/Hz. As we look at an example in the upcoming blogs you’ll see how easy it is to forget some of these finer details from time to time.
In any of these calculators only one parameter must be entered to calculate the other parameters. In this example, I simply input the NSD of an ADC in the dBm/Hz box for NSD and the tool (with an assumed 50 ohm system impedance) calculates the noise figure in dB and the noise spectral density in nV/ √ Hz. As an applications engineer I’ve found that different customers like to see different values – some prefer to look at noise figure while others prefer to operate with the noise spectral density in their signal chain analysis. This tool allows the user to operate with their parameter of choice when completing an analysis of a particular application.
When in doubt about the operation of the tool or when looking to find out more information on how calculations are made within the tool, the Help menu can be accessed. This menu has many different items to help the user understand the operation of the ADIsimRF tool as well as some examples. In addition there are some nice generally helpful tidbits of information.
Next time we’ll continue looking at the ADIsimRF tool specifically what modes are supported by the tool (and are selectable via the menu bar. We will also explore a signal chain example with a typical ADC circuit. We will see how the ADIsimRF tool can be used to quite accurately predict the performance of a signal chain without the need for obtaining and setting up hardware. Now if you are like myself, the goal of using the tool is more to set expectation for results that I intend to test in the lab, but I always enjoy getting to work in the lab collecting real data…and of course, I can obtain the necessary parts without much issue. For some out there it may not be as easy to obtain many different pieces of hardware to put a signal chain together so prototyping via a signal chain analysis tool like ADIsimRF opens up avenues for system design that may otherwise be unavailable due to budget constraints or for other reasons. I encourage you as the reader to ask questions if you would like to see something specific from the tool. Stay tuned as we continue looking at ADIsimRF and its capabilities.