In Analog Devices Design Tools: ADIsimRF Part 1 and Analog Devices Design Tools: ADIsimRF Part 2 of this series we looked at the Tools and Help menus in the ADIsimRF tool. As a reminder, this tool can be downloaded after filling out a brief software request form here: ADIsimRF. Now we will look into the signal chain portion of the tool and examine the modes of operation available, specifically the Rx mode since that includes ADCs. As I’ve mentioned in the previous parts of this series, the ADIsimRF tool has many different calculation possibilities which are very nice to have on the hand. These can be useful when either in the lab collecting data or when working on a report in your office. The tool outputs are great to double check experimental results or to set expectations when about to take data.
The ADIsimRF tool can be toggled between Tx and Rx modes. The Tx mode includes DACs and the Rx mode includes ADCs. Since I provide product support for high speed ADCs, we will focus on the Rx mode. The mode can be selected in two different ways as indicated below in the areas of the figure highlighted in red. By clicking on ‘Mode’ in the menu bar, the mode of the tool can be selected. The Toggle Tx/Rx button on the left of the window may also be used to select the desired mode.
Now let’s look at building a signal chain. In this example we will set up the signal chain given by CN-0242 titled “High Performance, High IF, 75 MHz Bandwidth, 14-bit, 250 MSPS Receiver Front End with Band-Pass Anti-aliasing Filter.” This circuit includes a balun, amplifier, anti-aliasing filter, and an ADC. In the circuit note a 1:3 impedance ratio balun is used which is not available in the ADIsimRF tool. In this example we will approximate with a 1:4 impedance ratio balun since that is available in the ADIsimRF component selection. To begin we select the appropriate number of stages required. In this case we need a total of four stages. By default, ADIsimRF has 10 available stages. To decrement the stages, select ‘Stage’ from the menu bar and decrement the stages to achieve the desired number.
Once the desired number of stages has been selected we can begin selecting the components in the signal chain. In this example Stage 1 is a balun, Stage 2 is a DGA (ADL5202), Stage 3 is a band-pass anti-aliasing filter, and Stage 4 is the AD9643-250 14-bit ADC. Select ‘Balun 1:4’ from the top drop down box and then select ‘Temp Part’ since the exact part number of the balun used in CN-0242 is not available. Next, fill in the Input Freq (MHz) = 180 MHz (center of the desired band), Zin (Ohms) = 50 ohms, Zout (Ohms) = 150 ohms, Power Gain (dB) = –0.7 dB and then select an input power in the ‘Input’ section at the bottom of the window (-14.2 dBm in this example).
Next, add the ADL5202 DGA into ‘Stage 2’. Once again, set the Input Freq (MHz) = 180 MHz, but in this case select only the Power Gain (dB) from the drop down box. For this example it is 19.5 dB. After selecting the gain, click outside the drop down box. The rest of the parameters for the DGA will auto-fill.
Next set ‘Stage 3’ to a BPF. Set the Input Freq (MHz) = 180 MHz, Zin (Ohms) = 100 ohms, Zout (Ohms) = 293 ohms, and Power Gain (dB) to -2.3 dB. All this information can be found in the circuit note.
The last and final stage to select is the ADC. In this example, the circuit note is using the dual 14-bit AD9643-250. Select ‘Dual ADC’ from the top drop down box and then select ‘AD9643-250’ from the second drop down box. Set the Input Freq (MHz) = 180 MHz and then click in another box which will then result in the rest of the parameters being auto-filled. The default impedance that is defined for an ADC in the tool is 200 ohms (Note: changing this will remove the ADC selection). For the purposes of this calculation this approximate impedance is sufficient.
Now we have all the stages defined for the circuit in ADIsimRF. We can now take a look at the parameters that the tool gives us. Notice all the parameters at the bottom of the ADIsimRF window. Specifically, let’s look at a few to compare to the results given in CN-0242. We have an analysis bandwidth of 125 MHz (which is defined by the Nyquist zone of the AD9643-250). The output NSD is -146.5 dBm/Hz. At first glance this looks incorrect, but don’t forget that the tool provides this number as an RF figure of merit rather than an ADC figure of merit. What I mean by this is that RF engineers typically look at units of dBm/Hz while an ADC guy such as myself looks at units of dBFS/Hz.
As I was going over this example with one of my colleagues who is more RF focused I spent some time thinking the tool was somehow incorrect before we realized the discrepancy was the dBm/Hz versus the dBFS/Hz. To do the conversion we can use the ADIsimRF calculator tool for Vrms, Vpp, dBV, dBm, mW. Since ADIsimRF uses 200 ohms for the input impedance of the ADC, set the ‘Resistance’ to 200 ohms. The AD9643-250 full scale input voltage is 1.75 Vpp so this is set in the Peak-to-Peak box. The results will then auto-fill. This gives us a result of a full scale input power of 2.8 dBm.
To go from units of dBm to dBFS simply “add” the 2.8 dBm to the results given in dBm to obtain units of dBFS. In this case the NSD is given as -146.5 dBm/Hz. Once we add 2.8 dBm to this number we obtain -149.3 dBFS/Hz for the overall NSD of the circuit. If we look at CN-0242 the measured SNR of the circuit is 68.4 dBFS. When converted to NSD, this yields -149.37 dBFS/Hz. I’d say this is pretty close to what the tool estimated since there is only 0.07 dBFS/Hz difference. Similarly, the reported SNR is 67.4 dB. To convert this to dBFS simply add 1 dBFS since the SNR is specified at an input power of -1 dBFS. This yields an SNR of 68.4 dBFS which is exactly as measured in CN-0242. What a nice result! It is great to have a tool that can quite accurately predict the results of a signal chain. I encourage readers out there to download the ADIsimRF tool and take advantage of a simple way to accurately predict the performance of a signal chain.