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AD7403BRIZ-RL7 Datasheet(PDF) 18 Page - Analog Devices

Part No. AD7403BRIZ-RL7
Description  16-Bit, Isolated Sigma-Delta Modulator
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Maker  AD [Analog Devices]
Homepage  http://www.analog.com
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AD7403BRIZ-RL7 Datasheet(HTML) 18 Page - Analog Devices

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Data Sheet
AD7403
APPLICATIONS INFORMATION
CURRENT SENSING APPLICATIONS
The AD7403 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 AD7403. The
AD7403 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
AD7403 are determined by the specific application require-
ments in terms of voltage, current, and power. Small resistors
minimize power dissipation, whereas low inductance resistors
prevent any induced voltage spikes, and good 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 AD7403, 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. The shunt current for a 3-phase induction
motor can be expressed as
PF
EF
V
P
I
W
RMS
×
×
×
=
73
.
1
where:
IRMS is the motor phase current (A rms).
PW is the motor power (Watts).
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 AD7403 (±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 31 shows the SINAD performance characteristics
and the ENOB of resolution for the AD7403 for different input
signal amplitudes. Figure 32 shows the rms noise performance
for dc input signal amplitudes. The performance of the AD7403
at lower input signal ranges allows smaller shunt values to be
used while still maintaining a high level of performance and
overall system efficiency.
VIN+ (mV)
60
65
70
75
80
85
90
0
50
100
150
200
250
11-BIT
ENOB
fIN = 1kHz
MCLKIN = 20MHz
VDD1 = 5V
VDD2 = 5V
TA = 25°C
12-BIT
ENOB
13-BIT
ENOB
14-BIT
ENOB
AD7403 20MHz
AD7403-8 20MHz
Figure 31. SINAD vs. VIN+ AC Input Signal Amplitude
0
0.2
0.4
0.6
0.8
1.0
1.2
1.4
1.6
–320
–240
–160
–80
0
80
160
240
320
MCLKIN = 5MHz
MCLKIN = 10MHz
MCLKIN = 20MHz
VIN+ DC INPUT SIGNAL AMPLITUDE (mV)
DC INPUT
100k SAMPLES PER DATA POINT
Figure 32. RMS Noise vs. VIN+ DC Input Signal Amplitude
RSHUNT must be able to dissipate the I2R power losses. If the
power dissipation rating of the resistor is exceeded, its 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 AD7403, 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 AD7403 can also be used for isolated voltage monitoring.
For example, in motor control applications, it can be used to
sense the bus voltage. In applications where the voltage being
monitored exceeds the specified analog input range of the
AD7403, a voltage divider network can be used to reduce the
voltage being monitored to the required range.
Rev. B | Page 17 of 24


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