ADUC7023 Analog Devices, ADUC7023 Datasheet - Page 88
Manufacturer Part Number
Precision Analog Microcontroller, 12-Bit Analog I/O, ARM7TDMI MCU
Specifications of ADUC7023
Mcu Speed (mips)
Adc # Channels
GROUNDING AND BOARD LAYOUT
As with all high resolution data converters, special attention
must be paid to grounding and PC board layout of the
ADuC7023-based designs to achieve optimum performance
from the ADCs and DACs.
Although the parts have separate pins for analog and digital
ground (AGND and DGND), the user must not tie these to two
separate ground planes unless the two ground planes are
connected very close to the part. This is illustrated in the
simplified example shown in Figure 50a. In systems where
digital and analog ground planes are connected together
somewhere else (at the system power supply, for example), the
planes cannot be reconnected near the part because a ground
loop would result. In these cases, tie all the ADuC7023 AGND
and DGND pins to the analog ground plane, as illustrated in
Figure 50b. In systems with only one ground plane, ensure that
the digital and analog components are physically separated onto
separate halves of the board so that digital return currents do
not flow near analog circuitry (and vice versa).
The ADuC7023 can then be placed between the digital and
analog sections, as illustrated in Figure 50c.
In all of these scenarios, and in more complicated real-life
applications, users should pay particular attention to the flow of
current from the supplies and back to ground. Make sure the
return paths for all currents are as close as possible to the paths
the currents took to reach their destinations.
For example, do not power components on the analog side (as
seen in Figure 50b) with IOV
currents from IOV
Figure 50. System Grounding Schemes
to flow through AGND. Avoid digital
because that would force return
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currents flowing under analog circuitry, which can occur if a
noisy digital chip is placed on the left half of the board (shown
in Figure 50c). If possible, avoid large discontinuities in the
ground plane(s) such as those formed by a long trace on the same
layer, because they force return signals to travel a longer path.
In addition, make all connections to the ground plane directly,
with little or no trace separating the pin from its via to ground.
When connecting fast logic signals (rise/fall time < 5 ns) to any of
the ADuC7023 digital inputs, add a series resistor to each
relevant line to keep rise and fall times longer than 5 ns at the
input pins of the part. A value of 100 Ω or 200 Ω is usually
sufficient enough to prevent high speed signals from coupling
capacitively into the part and affecting the accuracy of ADC
The clock source for the ADuC7023 can be generated by the
internal PLL or by an external clock input. To use the internal
PLL, connect a 32.768 kHz parallel resonant crystal between
XCLKI and XCLKO, and connect a capacitor from each pin to
ground, as shown in Figure 51. The crystal allows the PLL to lock
correctly to give a frequency of 41.78 MHz. If no external crystal
is present, the internal oscillator is used to give a typical
frequency of 41.78 MHz ± 3%.
To use an external source clock input instead of the PLL (see
Figure 52), Bit 1 and Bit 0 of PLLCON must be modified. The
external clock uses P1.1 and XCLK.
Using an external clock source, the ADuC7023 specified
operational clock speed range is 50 kHz to 44 MHz ± 1%, which
ensures correct operation of the analog peripherals and Flash/EE.
Figure 51. External Parallel Resonant Crystal Connections
Figure 52. Connecting an External Clock Source