EVAL-AD7787EB Analog Devices Inc, EVAL-AD7787EB Datasheet - Page 19

• Manufacturer Part Number: EVAL-AD7787EB
• Manufacturer: Analog Devices Inc

Specifications of EVAL-AD7787EB

• Lead Free Status / Rohs Status: Not Compliant

Reference voltage sources like those previously recommended,
e.g., ADR391, will typically have low output impedances and are,
therefore, tolerant to having decoupling capacitors on REFIN
without introducing gain errors in the system. Deriving the
reference input voltage across an external resistor means that
the reference input sees a significant external source impedance.
External decoupling on the REFIN pin is not recommended in
this type of circuit configuration.
V
Along with converting external voltages, the AD7787 can
monitor the voltage on the V
bits in the communications register are set to 1, the voltage on
the V
voltage is applied to the Σ-Δ modulator using an internal
useful because variations in the power supply voltage can be
monitored.
GROUNDING AND LAYOUT
The digital filter provides rejection of broadband noise on the
power supply, except at integer multiples of the modulator
sampling frequency. The digital filter also removes noise from
the analog and reference inputs, provided that these noise
sources do not saturate the analog modulator. As a result, the
AD7787 is more immune to noise interference than a
conventional high resolution converter. However, because the
resolution of the AD7787 is so high, and the noise levels from
the AD7787 are so low, care must be taken with regard to
grounding and layout.
The printed circuit board that houses the AD7787 should be
designed such that the analog and digital sections are separated
and confined to certain areas of the board. A minimum etch
technique is generally best for ground planes because it gives
the best shielding.
It is recommended that the AD7787’s GND pin be tied to the
AGND plane of the system. In any layout, it is important that
the user keep in mind the flow of currents in the system,
ensuring that the return paths for all currents are as close as
1.17 V reference for the analog-to-digital conversion. This is
DD
MONITOR
DD
pin is internally attenuated by 5 and the resultant
ATTENUATION
CIRCUIT
DD
pin. When the CH1 and CH0
12V OR 42V
(60V PEAK)
+
–
–200A TO
+2000A
Figure 16. Battery Monitoring
R
100µΩ
SHUNT
Rev. 0 | Page 19 of 20
AIN1(+)
AIN1(–)
AIN2
possible to the paths the currents took to reach their
destinations. Avoid forcing digital currents to flow through the
AGND sections of the layout.
The AD7787’s ground plane should be allowed to run under the
AD7787 to prevent noise coupling. The power supply lines to
the AD7787 should use as wide a trace as possible to provide
low impedance paths and reduce the effects of glitches on the
power supply line. 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. Avoid crossover of digital and analog
signals. Traces on opposite sides of the board should run at
right angles to each other. This reduces the effects of
feedthrough through the board. A microstrip technique is by far
the best, but it is not always possible with a double-sided board.
In this technique, the component side of the board is dedicated
to ground planes, while signals are placed on the solder side.
Good decoupling is important when using high resolution
ADCs. V
with 0.1 µF capacitors to GND. To achieve the best from these
decoupling components, they should be placed as close as
possible to the device, ideally right up against the device. All
logic chips should be decoupled with 0.1 µF ceramic capacitors
to DGND.
APPLICATIONS
Battery Monitoring
In battery monitoring, the battery current and voltage are
measured. The current is passed through a 100 µΩ resistor.
Because the current is from −200 A to +2000 A, the result is a
voltage from −20 mV to +200 mV. Channel AIN1 of the
AD7787 can be connected directly to the shunt resistor to
measure this current. The battery voltage can vary from 12 V to
42 V with peaks up to 60 V. This voltage is attenuated using an
external resistor network before being applied to the AD7787.
The buffers onboard the AD7787 mean that channel AIN2 can
be connected directly to the high impedance attenuator circuit
without introducing gain errors.
GND
V
DD
DD
MUX
should be decoupled with 10 µF tantalum in parallel
ADC
REFIN
Σ-∆
INTERFACE
CONTROL
AD7787
SERIAL
LOGIC
AND
DOUT/RDY
DIN
SCLK
CS
AD7787