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AD8212YRMZ-RL Arkusz danych(PDF) 10 Page - Analog Devices |
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AD8212YRMZ-RL Arkusz danych(HTML) 10 Page - Analog Devices |
10 / 16 page AD8212 Rev. 0 | Page 10 of 16 HIGH VOLTAGE OPERATION USING AN EXTERNAL PNP TRANSISTOR The AD8212 offers features that simplify measuring current in the presence of common-mode voltages greater than 65 V. This is achieved by connecting an external PNP transistor at the output of the AD8212 as shown in Figure 23. The VCE break- down voltage of this PNP becomes the operating common-mode range of the AD8212. PNP transistors with breakdown voltages exceeding 300 V, are readily available, inexpensive, and available in small packages. ROUT Q2 AD8212 BATTERY RSHUNT 1 8 5 2 3 OUTPUT CURRENT COMPENSATION BIAS CIRCUIT R1 R2 A1 Q1 VOUT RBIAS 6 Figure 23. High Voltage Operation Using External PNP The AD8212 features an integrated 5 V series regulator. This regulator ensures that at all times COM (Pin 2), which is the most negative of all the terminals, is always 5 V less than the supply voltage (V+). Assuming a battery voltage (V+) of 100 V, it follows that the voltage at COM (Pin 2) is (V+) – 5 V = 95 V The base emitter junction of Transistor Q2, in addition to the Vbe of one internal transistor, makes the collector of Transistor Q1 approximately equal to 95 V + 2(Vbe(Q2)) = 95 V + 1.2 V = 96.2 V This voltage appears across external Transistor Q2. The voltage across Transistor Q1 is 100 V – 96.2 V = 3.8 V In this manner, Transistor Q2 withstands 95.6 V and the internal Transistor Q1 is only subjected to voltages well below its breakdown capability. In this mode of operation, the supply current (IBIAS) of the AD8212 circuit increases based on the supply range and the RBIAS resistor chosen. For example: if V+ = 500 V and RBIAS = 500 kΩ IBIAS = (V+ − 5 V)/RBIAS then, IBIAS = (500 – 5)/500 kΩ = 990 μA In high voltage operation, it is recommended that IBIAS remain within 200 μA to 1 mA. This ensures that the bias circuit is turned on, allowing the device to function as expected. At the same time, the current through the bias circuit/regulator is limited to 1 mA. Refer to Figure 19 and Figure 21 for IBIAS and V+ information when using the AD8212 in a high voltage configuration. When operating the AD8212, as depicted in Figure 23, Transistor Q2 can be a FET or a bipolar PNP transistor. The latter is much less expensive, however the magnitude of IOUT conducted to the output resistor (ROUT) is reduced by the amount of current lost through the base of the PNP. This leads to an error in the output voltage reading. The AD8212 includes an integrated patented circuit, which compensates for the output current that is lost through the base of the external PNP transistor. This ensures that the correct transconductance of the amplifier is maintained. The user can opt for an inexpensive bipolar PNP, instead of a FET, while maintaining a comparable level of accuracy. OUTPUT CURRENT COMPENSATION CIRCUIT The base of the external PNP, Q2, is connected to ALPHA (Pin 6) of the AD8212. The current flowing in this path is mirrored inside the current compensation circuit. This current then flows in Resistor R2, which is the same value as Resistor R1. The voltage created by this current across Resistor R2, displaces the noninverting input of Amplifier A1 by the corresponding voltage. Amplifier A1 responds by driving the base of Transistor Q1 so as to force a similar voltage displacement across Resistor R1, thereby increasing IOUT. Since the current generated by the output compensation circuit is equal to the base current of Transistor Q2, and the resulting displacements across Resistor R1 and Resistor R2 result in equal currents, the increment of current added to the output current is equivalent to the base current of Transistor Q2. Therefore, the integrated output current compensation circuit has corrected IOUT such that no error results from the base current lost at Transistor Q2. This feature of the AD8212 greatly improves IOUT accuracy and allows the user to choose an inexpensive bipolar PNP (with low beta) with which to monitor current in the presence of high voltages (typically several hundred volts). |
Podobny numer części - AD8212YRMZ-RL |
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Podobny opis - AD8212YRMZ-RL |
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