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I'm making a board with a 8051 and 4 AD7705 to measure temperature with RTDs and I have some doubts about power supply. Should I have five 5V and GND connected in star (one for digital and 4 for ADCs)? Or 4 analog supplies, derived in four different points from the digital supply? Or just two supplies one for digital and one for analog? If you advise me to have 5 supplies, how should I connect them together? Because there is only one digital supply, the return path from each digital ADC signal should not affect the other ACD.
1) When splitting the ground plane into analogue and digital sections you are trying to ensure that digital current returns through the digital ground plane, analogue current returns through the analogue ground plane. 2) The two ground planes are connected together to ensure that the potential difference between AGND and DGND never exceeds +/-0.3V. If the potential difference between the ground planes exceeds this value then parasitic diodes and transistors can turn on in the ADC, if the current drawn is high enough, the IC will be permanently damaged. 3) Analogue ground and Digital ground are joined together at a single star point to avoid ground loops -(a ground loop is a situation where the digital return current modulates the analog return current, the ground return wire inductance and resistance is shared between the analog and digital circuits and this is what causes the interaction and resulting error. 4) The name DGND tells us that this pin connects to the digital ground of the IC. This does not imply that this pin must be connected to the digital ground of the system. In general these pins should be joined together, and to the analog ground of the system. It is not possible to join these two pins within the IC package, because the analog part of the converter cannot tolerate the voltage resulting from the digital currents flowing in the bond wire of the chip. Now what happens when you have multiple ADCs and DACs on a single board? Answer - implement a "star point" ground scheme. If you are using one power supply for both analogue and digital, then you can connect AGND and DGND of each ADC together underneath the device and provide a separate low impedance ground return path back to the star point from each ADC. From this star point, provide a low impedance path to the main off-board ground connection. The ADCs should be postioned as close together as possible, and the star point should be located close to the centre of all the ADCs. If you are using separate supplies with separate, electrically isolated, ground connections, then you should connect these grounds together at a single point as close possible to the ADCs. This then becomes the star point. Again position all the ADCs to be as close together as possible. For each ADC, provide a separate return path for DGND and AGND back to the star point. In this case, do not connect DGND and AGND together under each ADC as this will mean that Analogue ground and Digital ground are connected together at multiple points and you will get ground loops. -------------------------------------------------------------------------------- ---- In the case of the AD7705, as you have only one supply and ground reference point, you should locate all our ADC's as close as possible on the PCB board, each ADC should have it's own supply line, with 0.1uF decoupling caps located as close as possible to the ADC. In the case of a single PCB, refer to the fig 10.20 in our ch10 hardware design techniques of our mixed signal seminar book located at the following page. http://www.analog.com/support/standard_linear/seminar_material/mixedSignal/Mixed Signal_Sect10.pdf As you will see, the pcb board locates all the analog circuitry with it's analog ground plane on one side and the digital circuitry on the other side with it's digital ground plane. Each ADC will connect it's gnd pin to the analog ground plane. On page 10.31 we have included a grounding philosophy summary, which I suspect you will find useful.