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ADUM7703 Datasheet(PDF) 18 Page - Analog Devices |
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ADUM7703 Datasheet(HTML) 18 Page - Analog Devices |
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18 / 22 page  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. 0 50 100 150 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. R VIN– R VIN+ C C ADuM7703 Figure 29. RC Low-Pass Filter Input Network |
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