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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

RMS

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)

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

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

–320

–240

–160

–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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