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High accuracy shunt current sensor AD8205 and its application

Published on Jan 20 2011 // Current Sensor Circuits

 High accuracy shunt current sensor AD8205 and its application 1

AD8205 of Analog Devices has introduced a high-performance single-supply difference amplifier, a typical single-supply voltage of 5V, the input common-mode voltage range -2 ~ 65V, can be tolerated -5 ~ +70 V input common-mode voltage, high common mode voltage for detection of small differential voltage case of industrial equipment. It is a fixed gain of 50V / V, operating temperature range of -40 ~ +125 , offset voltage drift is less than 15µV , the gain drift is less than 30ppm (ambient temperature up to 125 ), the provisions in the temperature range with excellent DC performance, and its frequency band from DC to 100kHz range with up to 80dB common mode rejection ratio. Therefore, the measurement loop error, high precision, very suitable for motor control, drive control, magnetic control, vehicle dynamic control, fuel injection control, engine management and other DC-DC conversion control system.

Figure 1 high-side current sensor AD8205 internal circuit diagram
  Internal circuit structure and working principle
  AD8205’s internal circuitry from the A1 and A2 are two integrated operational amplifier and a resistor network and a small voltage reference and bias circuits, the circuit structure shown in Figure 1.
  A1 pre-attenuation by the resistor RA, RB, RC components, common-mode voltage can be attenuated to the proper input voltage range. Constitute a bridge network of two attenuator, the attenuation rate of 1/16.7. After the input signal is attenuated so that the amplitude of the input signal remain within the power supply voltage when the input voltage exceeds the power supply voltage or lower than the voltage of public places, the internal reference voltage work, making a total of amplifier negative input mode voltage signals can still work. When the bridge is balanced, common-mode voltage signal differential input signal is generated 0V. Of course, the input network also attenuates the input differential voltage signal, the amplifier A1 will be attenuated 26-fold signal amplification, and its inputs and outputs are differential forms of communication to get the most common mode rejection ratio. In addition resistance RA, RB, RC, RD and RF resistance after the laser calibration is better than 0.01% matching rate, this high-precision calibration allows the device to receive more than 80dB common mode rejection ratio.
  The output of amplifier A2 to A1 single-ended signals into differential signals, and zoom in 32.15 times. VREF1 and VREF2 reference input have been resistance RREF connected to the inverting input of A2, so that the output can be adjusted to the desired output voltage range. When the parallel use of two reference inputs, the reference voltage from input to output gain of 1V / V; when used alone when any one reference input, the gain of 0.5V / V. AD8205 total gain of the circuit by the attenuation decay rate 1/16.7, A1 and A2 in the magnification of the magnification of 26 constitute 32.15. AD8205 has a 300µA pull-down current of the absorption capacity, the use of Class A tube connected to the pull-up resistor PNP output.
  Output mode
  Unipolar Output
  This method is usually used to measure the flow through the sampling resistor current changes in one direction. There are two basic modes: a ground reference and the V + referenced output mode. In the unipolar mode, when the differential input is 0, the output can be offset to the negative (near ground) or positive peak (V +). When the differential input voltage is applied, the output will be moving back to the peak. Corresponding to full output when the input differential voltage amplitude close to 100mV, its output voltage polarity determined by the static setting. When the bias to the positive peak, the input differential voltage should be negative, the output decreased by a positive peak; the other hand, if the static bias to the ground, the input differential voltage should be positive, the output from 0 up.

 High accuracy shunt current sensor AD8205 and its application 2

(A) a ground reference (b) as a reference to V +
  Figure 2 unipolar output connections

High accuracy shunt current sensor AD8205 and its application 3

(A) high side current sensors and low-side switch means (b) high side current sensors and high-side switch means
  Figure 4, the circuit configuration
  A ground-referenced output connections as shown in Figure 2 (a) below. Its two reference inputs are received on the ground, when the input differential voltage is 0, the output is biased to the reverse peak (about 0.05V).
  To V + referenced output connections as shown in Figure 2 (b) below. Its two reference inputs are connected to the positive supply, when the input differential voltage is 0, the output is biased to the positive phase peak (about 4.8V).
  Bipolar Output
  AD8205 bipolar output can be measured on the flow through the bidirectional current sense resistor, then the output can be offset to the output anywhere within. When testing both positive and negative direction of the current amplitude, the output must be offset to the middle of full scale output. When the bi-directional current amplitude asymmetry, the output offset can be mapped to deviate from the semi-range position. VREF1 its two reference inputs and an internal resistor connected VREF2 RREF were received after the node with an internal bias, the two reference input mode of operation are identical. In the two reference voltage inputs access the corresponding voltage output of the bias can be completed. In the bipolar output mode, generally have the following two connections.
  1) When the input current amplitude is the same way, the two reference voltage inputs are connected to an external reference voltage source output, as shown in Figure 3 (a) shows connections. When the input voltage is negative relative to the-IN, the output voltage from the reference voltage drop. Conversely, when the input voltage is positive relative to the-IN, the output voltage from reference voltage to rise.
  2) the two reference voltage inputs, one receiving the supply voltage V + terminal, and the other reference voltage input is grounded, as shown in Figure 3 (b) shows connection. When the input differential voltage is 0, the output voltage is biased to the AD8205’s power supply voltage in the middle. The advantages of such a connection does not require external measurement of bi-directional current reference source, the output will automatically follow the ratio to changes in power supply voltage generated demi bias. In other words, whether the supply voltage rise or drop, the output bias point will remain in the middle of the supply voltage. For example, the power supply voltage of 5V, the output bias to 2.5V; and when the supply voltage increased by 10%, the output will be biased to 2.75V.
  Typical applications
  High-side current sensors and low-side switch means
  Figure 4 (a) shows connection, PWM control switch connected reference source, the inductive load and the sampling resistor in series connected between power supply and PWM control switch. When the PWM switch is closed, the sampling resistance of the common-mode voltage drops to near the negative peak; when the PWM switch is turned on, the sampling resistor common-mode voltage generated by power supply voltage and the freewheeling diode forward voltage drop and. Advantage of this approach is that when PWM switch is closed, the sample resistance on the power of high side current sense resistor circuit which still makes all the load current, including the continued flow of current, can still be monitored, and easy to identify the to short-circuit fault in order to achieve short-circuit protection circuit.
  High-side current sensors and high-side switch means
  Figure 4 (b) shows connection, PWM switch and sense resistor are located in the high voltage side. When the PWM switch is turned on, the load power will be removed, but still can provide and monitor the continued flow of current to achieve the current control diagnosis. In the course of their work, most of the power supply and load isolation, so that the load on the ground can be the difference between the adverse effects caused by the voltage reduced to a minimum. When the PWM switch is closed, the supply voltage will be connected to the load, when the common-mode voltage will increase power supply voltage. The PWM switch is turned on, the voltage will be reversed and after inductive load, the role of the freewheeling diode, making the sampling resistor to maintain a common-mode voltage to the diode below the conduction voltage drop.

High accuracy shunt current sensor AD8205 and its application 4

Figure 5 motor control schematic
  Motor Control
  Connected as shown in Figure 5, AD8205 in the H bridge motor control circuit as part of the control circuit, motor and serial sampling resistor placed in the H after the middle of the bridge, by detecting the voltage on the sampling resistor, can accurately measure the motor current current and its direction. At this point, AD8205 output is set to external reference two-way manner, so that it can measure bi-directional switch current H bridge and also monitors the motor’s running direction. Not a particularly stable due to the reference level, a ground reference will result in measurement inaccuracies. Therefore, this test program than to ground level as a reference test method much better.
  Conclusion
  In short, the use of AD8205 to achieve high common-mode voltage in the case of small differential voltage detection circuit structure is simple and reliable monitoring of high accuracy. It is especially suitable for 42V automotive system power control, hydraulic control, control of magnetic suspension control system.

High accuracy shunt current sensor AD8205 and its application 5