As I thought about where to go with the next blog, I looked over the comments and questions from my previous blogs to see where you as the reader had questions. It seems we have taken a pretty long journey discussing the various aspects of noise pertaining to ADCs. It became apparent that it would be a good time to begin looking at interfacing to an ADC through these various ports: power supplies, ground, analog inputs, clock input, and digital I/O. This will obviously intertwine with some of the noise discussion we’ve had, but with a little more practical application in mind. We’ll start by taking a look at the power supply inputs of the ADC, what we typically use to supply power to the ADC, and some of the tradeoffs of the different methods.
Before we discuss how to drive the various supply domains, let’s review the power supply inputs that can typically be found on a high-speed ADC. There is an optional input buffer supply domain (not on all ADCs), an analog supply domain, a digital supply domain, and a driver supply domain.
Depending on the ADC, an analog input buffer may be used in front of the ADC analog inputs to the ADC core. In some applications, it can be beneficial to have an input buffer since most high-speed ADCs have a switched capacitor input stage. We won’t dive into a discussion on this topic other than in regard to the power supply requirements when a buffer is present.
Typically, the data conversion (analog section) and digital processing (digital section) are maintained on different supply inputs. The analog section comprises multiple stages of amplifiers, comparators, and other analog circuitry that does most of the analog-to-digital conversion. One could definitely argue that this section isn’t truly purely analog, and one would be correct. Many high-speed converters today have digitally assisted analog sections. But then again, one could also argue that there are no such things as digital circuits since it is all made from analog circuits in the end, but I digress… back to the analog power supply.
Typically the analog supply has its own input for isolation reasons. The same applies to the digital section. There are various portions of circuitry within each of these blocks that operate at different frequencies, and keeping the supply domains separate helps keep these frequencies from making their way back and forth between the two sections. Having crosstalk between these sections can result in degraded performance, which may manifest itself in the SNR (noise performance) or the SFDR (spurious performance) of the ADC.
In a similar manner, the driver supply domain is typically kept separate from the other supply domains of the converter. The output drivers, depending on the particular type, can be a potential pathway for noise into the converter or may potentially be a source of noise. For example, in ADCs with CMOS outputs, large switching transients could create noise and in particular instances could cause performance issues with the ADC. For CML output drivers that are typically used with high-speed serial ADCs that employ JESD204B outputs, it is best to keep a separate domain to ensure an optimal supply condition for the high output data rates that are required (up to 12.5 Gbit/s).
Stay tuned as we continue to look at the power supply inputs of a typical high-speed ADC. Now that we have an understanding of the various domains, we can start looking at strategies for powering them. Do we exclusively use LDOs? There can be some cases where LDOs can be very appropriate to use and others where they may not be the best option. Can we use a DC/DC converter? It depends. Sometimes, DC/DC converters are the most efficient way to translate supply domains down from a high input voltage. Why not use a combination of both? Now, here we may be on to something! Do we need to use one versus the other?
These are some questions we will explore. I hope you are excited to find out the answers. I am looking forward to discussing them!