AD7402 datasheet(15/20 Pages) AD

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Data Sheet

AD7402

Rev. A | Page 15 of 20

APPLICATIONS INFORMATION

CURRENT SENSING APPLICATIONS

The AD7402 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 AD7402. The

AD7402 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

AD7402 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 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 AD7402, 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

IRMS =



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 AD7402 (±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 26 shows the SINAD performance characteristics

and the ENOB of resolution for the AD7402 for different input

signal amplitudes. Figure 27 shows the rms noise performance

for dc input signal amplitudes. The AD7402 performance at

lower input signal ranges allows smaller shunt values to be used

while still maintaining a high level of performance and overall

system efficiency.

Figure 26. SINAD vs. VIN+ AC Input Signal Amplitude

Figure 27. 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 AD7402, 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 AD7402 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

AD7402, 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 AD7402 can be connected directly across the

shunt resistor with a simple RC low-pass filter on each input.

VIN+ AC INPUT SIGNAL AMPLITUDE (mV)

100

150

200

250

14-BIT

ENOB

13-BIT

ENOB

11-BIT

ENOB

fIN = 35Hz

VDD1 = 5V

VDD2 = 5V

TA = 25°C

12-BIT

ENOB

RMS NOISE (LSB)

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

1.8

2.0

–250

–170

–90

–10

150

230

DC INPUT

100k SAMPLES PER DATA POINT

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