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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.

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.

VIN–

VIN+

ADuM7703

Figure 29. RC Low-Pass Filter Input Network

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