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the DS recommends (p. 25 of rev0) 10uF tantalum // 100nF ceramic. For availability, reliability and size reasons I would like to avoid tantalum. What do you think of a 10uF X5R ceramic instead of the recommended combination? What if I add a maybe 10 ohms resistor in series to create a further LP pole? Ground return: what do you think of a central ground point on the track connecting GND and PWRGND, where the negative from the regulator and referece chips would be connected?
On most of our converters we recommend a 10uF tantal // 100nF chip ceramic combination. This particular combination gives excellent decoupling across a broad band of frequencies. The large value cap decouples low frequency ripple, but most large value caps have a transition frequency in the region of hundreds of kilohertz where parasitic effects take over and they start to look inductive. The 100nF chip ceramic has a higher transition frequency (due mostly to it’s smaller physical size) and decouples the higher frequencies. The optimum choice of decoupling cap combination is highly application specific and depends on power supply ripple, board parasitics, noise, conducted and radiated interferance coupling onto the power line, and ground plane impedance, however this combination has shown over time that it provides adequate decoupling in a large variety of applications. The choice of a 10uF tantal vs. a 10uF X5R is not critical from the point of view of the AD7709 ADC, but it may be critical for your power supply or the particular LDO you choose. If you use one of ADIs “anyCap” LDO such as the ADP3330, it will be insensitive to the differences in ESR of the two caps. Most critical to the correct application of the AD7709 is that the decoupling caps are mounted as close as possible to power pins and the ground pins has a direct low impedance return path back to the power supply. For example narrow via from the negative side of the decoupling cap to a buried ground plane can add inductance. The series resistor can be useful in some applications but again this really depends on just how much ripple you have on your supply and how much interference you anticipate. Just one word of warning about using series resistors on the power supply, if you have a current system, you can get quite a voltage drop across this resistor. Ensure that the inputs and digital IO lines of the ADC never see a voltage that is output the power supply pins of the ADC. “Ground return: what do you think of a central ground point on the track connecting GND and PWRGND, where the negative from the regulator and referece chips would be connected?” We recommend that GND and PWRGND are connected together directly at the IC and a low impedance ground return path is provided back to the regulator and system power supply ground. In most applications the Low side power switches will be in a steady state when measurements are made and the current flowing through the switches is relatively low. The only time that separate ground return paths for GND and PWRGND would make sense is if the PWRGND return current was high and had a high frequency content. However, in most applications the PWRGND will essential be DC in nature and current <<20mA.