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ADUM7703 Datasheet(PDF) 18 Page - Analog Devices

Part No. ADUM7703
Description  16-Bit, Isolated, Sigma-Delta ADC
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Manufacturer  AD [Analog Devices]
Direct Link  http://www.analog.com
Logo AD - Analog Devices

ADUM7703 Datasheet(HTML) 18 Page - Analog Devices

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ADuM7703
Data Sheet
Rev. 0 | Page 18 of 22
APPLICATIONS INFORMATION
CURRENT SENSING APPLICATIONS
The ADuM7703 is ideally suited for current sensing applications
where the voltage across a shunt resistor (RSHUNT) is monitored.
The load current flowing through an external shunt resistor
produces a voltage at the input terminals of the ADuM7703.
The ADuM7703 provides isolation between the analog input
from the current sensing resistor and the digital outputs. By
selecting the appropriate shunt resistor value, a variety of current
ranges can be monitored.
Choosing RSHUNT
The shunt resistor (RSHUNT) values used in conjunction with
the ADuM7703 are determined by the specific application
requirements in terms of voltage, current, and power. Small
resistors minimize power dissipation, whereas low inductance
resistors prevent any induced voltage spikes, and high tolerance
devices reduce current variations. The final values chosen are
a compromise between low power dissipation and accuracy.
Higher value resistors use the full performance input range of
the ADC, thus achieving maximum SNR performance. Low
value resistors dissipate less power but do not use the full
performance input range. The ADuM7703, however, delivers
excellent performance, even with lower input signal levels,
allowing low value shunt resistors to be used while maintaining
system performance.
To choose a suitable shunt resistor, first determine the current
through the shunt. Calculated the shunt current for a 3-phase
induction motor as
IRMS = PW/(1.73 × V × EF × PF)
where:
IRMS is the motor phase current (A rms).
PW is the motor power (W).
V is the motor supply voltage (V ac).
EF is the motor efficiency (%).
PF is the power efficiency (%).
To determine the shunt peak sense current (ISENSE), consider the
motor phase current and any overload that may be possible in
the system. When the peak sense current is known, divide the
voltage range of the ADuM7703 (±250 mV) by the peak sense
current to yield a maximum shunt value.
If the power dissipation in the shunt resistor is too large, the
shunt resistor can be reduced and less of the ADC input
range can be used. Figure 28 shows the SINAD performance
characteristics and the ENOB of resolution for the ADuM7703
for different input signal amplitudes. The performance of the
ADuM7703 at lower input signal ranges allows smaller shunt
values to be used while still maintaining a high level of
performance and overall system efficiency.
0
50
100
150
VIN+ (mV)
200
250
fIN = 1kHz
MCLKIN = 20MHz
VDD1 = 5V
VDD2 = 3V
TA = 25°C
14-BIT
ENOB
13-BIT
ENOB
12-BIT
ENOB
11-BIT
ENOB
Figure 28. SINAD vs. VIN+ AC Input Signal Amplitude
RSHUNT must dissipate the current2 × resistance (I2R) power
losses. If the power dissipation rating of the resistor is exceeded,
the value may drift, or the resistor may be damaged, resulting in
an open circuit. This open circuit can result in a differential
voltage across the terminals of the ADuM7703, in excess of the
absolute maximum ratings. If ISENSE has a large high frequency
component, choose a resistor with low inductance.
VOLTAGE SENSING APPLICATIONS
The ADuM7703 can also be used for isolated voltage
monitoring. For example, in motor control applications, the
device can be used to sense the bus voltage. In applications where
the voltage being monitored exceeds the specified analog input
range of the ADuM7703, a voltage divider network can be used
to reduce the voltage being monitored to the required range.
INPUT FILTER
In a typical use case for directly measuring the voltage across a
shunt resistor, the ADuM7703 can be connected directly across
the shunt resistor with a simple RC low-pass filter on each input.
The recommended circuit configuration for driving the differential
inputs to achieve best performance is shown in Figure 29. An
RC low-pass filter is placed on both the analog input pins.
Recommended values for the resistors and capacitors are 10 Ω and
220 pF, respectively. If possible, equalize the source impedance
on each analog input to minimize offset.
R
VIN–
R
VIN+
C
C
ADuM7703
Figure 29. RC Low-Pass Filter Input Network


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