adsst-em-3035k Analog Devices, Inc., adsst-em-3035k Datasheet - Page 12

• Manufacturer Part Number: adsst-em-3035k
• Description: Salem Three-phase Electronic Energy Meter
• Manufacturer: Analog Devices, Inc.

ADSST-EM-3035
Grounding and Layout
Since the analog inputs to the ADSST-73360AR are differential,
most of the voltages in the analog modulator are common-mode
voltages. The excellent common-mode rejection of the part will
remove common-mode noise on these inputs. The analog and
digital supplies of the ADSST-73360AR are independent and
separately pinned out to minimize coupling between analog and
digital sections of the device. The digital filters on the encoder
section will provide rejection of broadband noise on the power
supplies, except at integer multiples of the modulator sampling
frequency. The digital filters also remove noise from the analog
inputs provided the noise source does not saturate the analog
modulator. However, because the resolution of the ADSST-
73360LAR ADC is high, and the noise levels from the
ADSST-73360AR are so low, care must be taken with regard to
grounding and layout.
The printed circuit board that houses the ADSST-73360AR
should be designed so the analog and digital sections are sepa-
rated and confined to certain sections of the board. The
ADSST-73360AR pin configuration offers a major advantage in
that its analog and digital interfaces are connected on opposite
sides of the package. This facilitates the use of ground planes
that can be easily separated, as shown in Figure 5. A minimum
etch technique is generally best for ground planes as it gives the
best shielding. Digital and analog ground planes should be
joined in only one place. If this connection is close to the device,
it is recommended to use a ferrite bead inductor as shown in
Figure 5.
Avoid running digital lines under the device for they will couple
noise onto the die. The analog ground plane should be allowed
to run under the ADSST-73360AR to avoid noise coupling.
The power supply lines to the ADSST-73360AR should use as
large a trace as possible to provide low impedance paths and
reduce the effects of glitches on the power supply lines. Fast
switching signals such as clocks should be shielded with digital
ground to avoid radiating noise to other sections of the board,
and clock signals should never be run near the analog inputs.
Traces on opposite sides of the board should run at right angles
to each other. This will reduce the effects of feedthrough
through the board. A micro-strip technique is by far the best but
is not always possible with a double-sided board. In this tech-
nique, the component side of the board is dedicated to ground
planes while signals are placed on the other side.
Good decoupling is important when using high speed devices.
All analog and digital supplies should be decoupled to AGND
Figure 5. Grounding and Layout
ANALOG GROUND
DIGITAL GROUND
–12–
and DGND respectively, with 0.1 µF ceramic capacitors in
parallel with 10 µF tantalum capacitors. To achieve the best
from these decoupling capacitors, they should be placed as close
as possible to the device, ideally right up against it. In systems
where a common supply voltage drives both the AVDD and
DVDD of the ADSST-73360AR, it is recommended that the
system’s AVDD supply be used. This supply should have the
recommended analog supply decoupling between the AVDD
pins of the ADSST-73360AR and AGND and the recommended
digital supply decoupling capacitors between the DVDD pin
and DGND.
NOTE: FOR MORE DETAILS ON ADSST-73360AR, PLEASE
REFER TO DATA SHEET OF AD73360
Interfaces between ADSST-EM-3035 and Microcontroller
Overview
The following paragraphs describe the interface between the
ADSST-EM-3035 chipset and the microcontroller. The sequence
of operations is a critical issue for proper functioning of the two
processors on the board. The DSP processor is primarily used to
compute various parameters, provide the impulse outputs on the
external LEDs and provide automatic gain switching inside the
ADC. The microcontroller can collect the data from the chipset
for data management for further processing. There are two basic
functions that the microcontroller performs in a handshaking
mode with the DSP processor:
•
•
This section describes the Boot loading and SPI operations.
BOOT LOADING THE DSP PROCESSOR FROM THE
MICROCONTROLLER
The DSP processor has an internal program memory RAM that
supports boot loading. With boot loading, the processor reads
instructions from a byte-wide data bus connected to the microcon-
troller and stores the instructions in the 24-bit wide internal
program memory. The host microcontroller, is the source of
bytes to be loaded into on-chip memory. The choice of which
technique to use depends upon the I/O structure of the host
microcontroller, availability of I/O port lines, and the amount
of address decoding logic already available in the system. The
description here is one of the many ways that this could be
configured. However, the software on the microcontroller has
been written in way to make optimum use of the configuration.
Figure 6 illustrates the system implementation to allow a
microcontroller to boot the DSP processor. The only hardware
required is a D-type flip-flop and a 5 kΩ resistor. The resistor
is used to pull the DSP processor’s BMS pin (Boot Memory
Select) high.
The DSP processor boots using the BDMA option. The BDMA
option can be used when pins Mode A, Mode B, and Mode C on
the DSP are tied low. With these pins tied low the DSP auto-
matically enters its boot sequence after the processor is reset.
Boot loading the DSP with metering software on power up
(for non-ROM coded version only)
Communication with the DSP on SPI to:
Send Initialization data on power up after boot loading the
DSP with metering software
Receive data from DSP during normal operation
Receive and send data during calibration
REV. 0