EVAL-ADUC7026QSP Analog Devices Inc, EVAL-ADUC7026QSP Datasheet - Page 85

• Manufacturer Part Number: EVAL-ADUC7026QSP
• Description: KIT DEV FOR ADUC7026/7027
• Manufacturer: Analog Devices Inc

Specifications of EVAL-ADUC7026QSP

• Lead Free Status / RoHS Status: Contains lead / RoHS non-compliant

GROUNDING AND BOARD LAYOUT
RECOMMENDATIONS
As with all high resolution data converters, special attention
must be paid to grounding and PC board layout of the
ADuC7019/20/21/22/24/25/26/27/28-based designs to
achieve optimum performance from the ADCs and DAC.
Although the parts have separate pins for analog and digital
ground (AGND and IOGND), 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 78a. In systems where
digital and analog ground planes are connected together
somewhere else (at the system’s power supply, for example), the
planes cannot be reconnected near the part because a ground
loop would result. In these cases, tie all the ADuC7019/20/21/
22/24/25/26/27/28 AGND and IOGND pins to the analog
ground plane, as illustrated in Figure 78b. 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 ADuC7019/20/21/22/24/25/26/27/28 can then be placed
between the digital and analog sections, as illustrated in Figure 78c.
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.
b.
a.
c.
PLACE ANALOG
COMPONENTS
COMPONENTS HERE
COMPONENTS HERE
PLACE ANALOG
PLACE ANALOG
HERE
Figure 78. System Grounding Schemes
AGND
AGND
DGND
COMPONENTS HERE
COMPONENTS HERE
COMPONENTS HERE
PLACE DIGITAL
PLACE DIGITAL
PLACE DIGITAL
DGND
DGND
Rev. B | Page 85 of 92
For example, do not power components on the analog side (as
seen in Figure 78b) with IOV
currents from IOV
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 78c). 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 ADuC7019/20/21/22/24/25/26/27/28 digital inputs, add a
series resistor to each relevant line to keep rise and fall times
longer than 5 ns at the part’s input pins. 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 conversions.
CLOCK OSCILLATOR
The clock source for the ADuC7019/20/21/22/24/25/26/27/28 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 79. 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 80), Bit 1 and Bit 0 of PLLCON must be modified.The
external clock uses P0.7 and XCLK.
Using an external clock source, the ADuC7019/20/21/22/24/
25/26/27/28’s specified operational clock speed range is 50 kHz
to 44 MHz ± 1%, which ensures correct operation of the analog
peripherals and Flash/EE.
ADuC7019/20/21/22/24/25/26/27/28
Figure 79. External Parallel Resonant Crystal Connections
Figure 80. Connecting an External Clock Source
12pF
12pF
EXTERNAL
SOURCE
CLOCK
DD
32.768kHz
to flow through AGND. Avoid digital
XCLKO
XCLKO
XCLKI
XCLKI
XCLK
DD
45
44
because that would force return
ADuC7026
ADuC7026
TO
FREQUENCY
DIVIDER
TO
INTERNAL
PLL