In the last blog I introduced MOTIF (Modeling Operation Through the Implementation of Features) which is in the new simulator engine residing inside of VisualAnalog that is replacing ADIsimADC. As I mentioned, the purpose of the tool remains the same while the implementation of the simulator is getting an update. For more information on MOTIF please visit this site. Let’s now take a look at the various pieces of the puzzle that we put together to perform a simulation of an ADC using MOTIF in VisualAnalog.
First, to make any ADC operate we need to set up the input stimuli. The Tone Generator block (Figure 1) sets up the sample frequency, FFT depth (number of samples), analog input amplitude, and analog input frequency. If more than one input tone is used, the “Use Composite Amplitude” box can be checked so that the overall amplitude of the input signals equals what is entered in the form for the signal amplitude. These tones can be entered into the frequency table by clicking the “Add” button. Notice that the frequency is input in the “Frequency (MHz)” column and the coherent frequency is calculated automatically and filled in the “Actual (MHz)” column. To disable the use of a coherent analog input signal, select the checkbox “Non-integer Cycles.” If desiring to operate with a complex input signal check the “Complex” box.
Once the input stimuli and conditions are set using the Tone Generator, the next step is to configure the MOTIF ADC model settings. In our example in the last blog we took the simplest case for the AD9680 which was leaving the model settings in the default conditions where we just looked at the ADC only (without any of the DDC functions enabled). To access these settings click on the icon (circled in red in Figure 2) in the ADC Model: AD9680_1000 block within VisualAnalog .
The ADC Model Settings has two tabs for settings; General and Simulation. On the General tab (shown in Figure 3), the model properties are displayed along with the center frequency (which matches the Tone Generator). Optionally, the Nyquist Zone can be specified. The sets the input tone in the specified Nyquist zone where it aliases to the specified frequency. In addition, external jitter can be specified to view the effects of clock jitter on the ADC performance.
On the Simulation tab in the MOTIF ADC model settings (shown in Figure 4), the available device configurations can be selected. In the case of the AD9680 in this example there are various DDC (digital down converter) options that can be enabled in addition to the ADC only mode that is default. In this case we will look at the option Real ADC + DDC + C2R. This option sets the model to operate with a real input signal, enables the DDC, and enables the complex to real (C2R) block. The available options in this mode for the AD9680 include: DDC decimation rate (1, 2, 4, or 8), DDC gain (0 or 6), NCO frequency (valid between –fclk/2 and fclk/2), and NCO phase offset (valid between 0 and 360 degrees). In this example we will set the decimation rate to 1 (no decimation), the DDC gain to 0, the NCO frequency to 250MHz (centers band in first Nyquist zone), and the NCO phase to 0 degrees.
Once the model parameters are all set, click the run button in VisualAnalog to simulate the AD9680 with the settings that have been configured. This will generate an FFT plot like the one shown in Figure 5.
Now that we have simulated the AD9680 under the specified conditions, let’s once again take a look at an actual measurement to see how things line up. As you can see from Figures 5 and 6, the simulated data lines up quite well to the measured data.
If more information is desired on setting up VisualAnalog to use the MOTIF models (previously referred to as ADIsimADC models) this can be found here: AN-905. Thus far, we have looked exclusively at how MOTIF is used within VisualAnalog. The simulator engine is also employed in an online evaluation tool that can be accessed from the Analog Devices website. This allows access to simulate various Analog Devices high speed ADCs from the web without the need to install VisualAnalog. This can be useful if the user desires to quickly perform a simulation or if the user does not have installation privileges on their computer. Stay tuned as we continue looking at MOTIF to simulate ADC performance.