Q
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.
A
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.