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AD8207WBRZ Datasheet(PDF) 10 Page - Analog Devices |
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AD8207WBRZ Datasheet(HTML) 10 Page - Analog Devices |
10 / 16 page AD8207 Rev. 0 | Page 10 of 16 THEORY OF OPERATION The AD8207 is a single-supply, zero drift, difference amplifier that uses a unique architecture to accurately amplify small differential current shunt voltages in the presence of rapidly changing common-mode voltage. In typical applications, the AD8207 is used to measure current by amplifying the voltage across a shunt resistor connected to its inputs. The AD8207 includes a zero-drift amplifier, a precision resistor network, a common-mode control amplifier, and a precision reference (see Figure 25). A set of precision-trimmed resistors make up the network that attenuates the input common-mode voltage to within the supply range of the amplifier, in this case with a ratio of 20/1. This attenuation ensures that when the input pins are externally at the common-mode extremes of −4 V and +65 V, the actual voltage at the inputs of the main amplifier is still within the supply range. The input resistor network also attenuates normal (differential) mode voltages. Therefore, the total internal gain of the AD8207 is set to 400 V/V to provide a total system gain of 20 V/V. Total Gain (V/V) = 1/20 (V/V) × 400 (V/V) = 20 V/V The AD8207 is designed to provide excellent common-mode rejection, even with PWM common-mode inputs that can change at very fast rates, for example, 1 V/ns. An internal common-mode control amplifier is used to maintain the input common mode of the main amplifier at 3.5 V (with 5 V supply), and therefore eliminates the negative effects of such fast- changing external common-mode variations. The AD8207 features an input offset drift of less than 500 nV/°C. This performance is achieved through a novel zero-drift architecture that does not compromise band- width, which is typically rated at 150 kHz. The reference inputs, VREF1 and VREF2, are tied through 100 kΩ resistors to the positive input of the main amplifier, which allows the output offset to be adjusted anywhere in the output operating range. The gain is 1 V/V from the reference pins to the output when the reference pins are used in parallel. When the pins are used to divide the supply, the gain is 0.5 V/V. The AD8207 offers breakthrough performance without compromising any of the robust application needs typical of solenoid or motor control. The part rejects PWM input common-mode voltages, while the zero-drift architecture yields the lowest offset and offset drift performance on the market. ZERO-DRIFT AMPLIFIER AD8207 COMMON-MODE CONTROL AMPLIFIER 3.5V/2.2V REF 60kΩ 120kΩ SHUNT 120kΩ +IN GND –IN OUT VREF1 VREF2 60kΩ 9kΩ 50kΩ 6kΩ 6kΩ 100kΩ 100kΩ 100kΩ 100kΩ 100kΩ Figure 25. Simplified Schematic |
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