Electronic Components Datasheet Search 

AD7403BRIZRL7 Datasheet(PDF) 18 Page  Analog Devices 

AD7403BRIZRL7 Datasheet(HTML) 18 Page  Analog Devices 
18 / 25 page 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 3phase 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 11BIT ENOB fIN = 1kHz MCLKIN = 20MHz VDD1 = 5V VDD2 = 5V TA = 25°C 12BIT ENOB 13BIT ENOB 14BIT ENOB AD7403 20MHz AD74038 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 
