We have been considering various topics pertaining to ADC power supply inputs over the last several blogs, so let’s continue in that direction and keep expanding on the topic just a bit.
Thus far, we’ve looked at the different types of power supply inputs to an ADC and then a few ways we can drive them. Mainly we have focused on using LDOs, but we have seen this may not always be the best approach. Depending on system constraints and performance specifications, other topologies might be better. In that vein, let us look at using a DC/DC converter (sometimes referred to as a switching regulator) along with an LDO to drive the ADC power supply inputs (see Figure 1).
When using a DC/DC converter it is important to make sure that the output LC filter is properly designed for the current requirements of the design and the switching frequency of the DC/DC converter. It is also important to keep the current return loops in the current switching paths very short and tight around the DC/DC converter. We will keep this part of the discussion high-level and talk a bit about the effects that may show up in the FFT of the digitized data of the ADC.
First let’s look at the power consumption, as we have done in the last few blogs. In this case we will assume a 5.0 V supply rail as an input and that we are using the ADP2114 DC/DC converter and the ADP1741 LDO. In order to calculate the power dissipation of the ADP2114 we can download the ADIsimPower tool for the device. This tool will help us calculate the power dissipated by the ADP2114 as well as generate a schematic and design. For this example, we will only look at the power calculated by the tool.
Let’s again consider the AD9250 where the total current requirement for the device is 395 mA with an input supply voltage of 5.5 V, an output voltage of 2.5V, and the selection for “Most Efficient” design in the tool (see Figure 2 below).
Entering these values into the ADIsimPower tool for the ADP2114, the results are computed, which show an efficiency of 96.4% with a power dissipation of 37 mW. This is quite a bit more efficient than the previous examples we’ve looked at! This is exactly one of the reasons a DC/DC converter can be attractive. Finishing out this example, let’s now calculate the power dissipation in the ADP1741 now that we have a 2.5 V supply voltage available from the ADP2114.
In this case, the ADP1741 will be required to dissipate (2.5 V – 1.8 V) x 395mA = 276.5 mW. The means the maximum junction temperature Tj would be equal to TA + Pd x θ ja = 85o C + 276.5 mW x 42o C/W = 96.61o C, which is considerably less than the maximum rated junction temperature of 150o C for the ADP1741. This is a much better operating condition than in our previous examples. So what is the catch? Well, there are definitely things to consider when using a DC/DC converter. Since the DC/DC converter is a switching device, there are switching transients to be aware of that can manifest as spurs in the ADC output spectrum (see Figure 3).
The placement of the switching of these switching spurs will be dependent upon the switching frequency of the DC/DC converter and the input frequency of the ADC. The switching spurs will mix with the input signal, and spurs will result at fIN – fSW and fIN + fSW . The good news is that, with proper design, the amplitude of these spurs can be minimized and in many cases be lower than the harmonics or other spurs in the ADC spectrum, making them a non-issue. The ADIsimPower tool provides a schematic as well as a recommended layout so that the user can have an optimized design to minimize the effects of the switching action of the DC/DC converter (see Figures 4 and 5).
I encourage the use of the ADIsimPower tool. It is quite handy when looking to generate a power supply design for a system. We have used the tool here for an ADC, but the tool’s use is not limited to just ADCs. Stay tuned as we continue to look at ADC power supplies. Thanks again for the great comments and questions — please keep those coming.