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AD8205WHRZ-RL Datasheet(PDF) 8 Page - Analog Devices |
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AD8205WHRZ-RL Datasheet(HTML) 8 Page - Analog Devices |
8 / 12 page AD8205 Data Sheet Rev. | Page 8 of 12 THEORY OF OPERATION The AD8205 is a single-supply difference amplifier that uses a unique architecture to accurately amplify small differential current shunt voltages in the presence of rapidly changing common-mode voltages. In typical applications, the AD8205 is used to measure current by amplifying the voltage across a current shunt placed across the inputs. The gain of the AD8205 is 50 V/V, with an accuracy of 1.2%. This accuracy for the Y and W Grade is guaranteed over the operating temperature range of −40°C to +125°C. The H Grade version of the AD8205 is specified for operation from −40°C to +150°C. The input offset is less than 2 mV referred to the input at 25°C, and 4.5 mV maximum referred to the input over the full operating temperature range for the packaged part. The AD8205 operates with a single supply from 4.5 V to 10 V (absolute maximum = 12.5 V). The supply current is less than 2 mA. High accuracy trimming of the internal resistors allows the AD8205 to have a common-mode rejection ratio better than 78 dB from dc to 20 kHz. The output offset can be adjusted from 0.05 V to 4.8 V (V+ = 5 V) for unipolar and bipolar operation. The AD8205 consists of two amplifiers (A1 and A2), a resistor network, small voltage reference, and a bias circuit (not shown), see Figure 13. The set of input attenuators preceding A1 consist of RA, RB, and RC, which reduce the common-mode voltage to match the input voltage range of A1. The two attenuators form a balanced bridge network. When the bridge is balanced, the differential voltage created by a common-mode voltage is 0 V at the inputs of A1. The input attenuation ratio is 1/16.7. The combined series resistance of RA, RB, and RC is approximately 200 kΩ ± 20%. By attenuating the voltages at Pin 1 and Pin 8, the A1 amplifier inputs are held within the power supply range, even if Pin 1 and Pin 8 exceed the supply or fall below common (ground). A reference voltage of 250 mV biases the attenuator above ground. This allows the amplifier to operate in the presence of negative common-mode voltages. The input network also attenuates normal (differential) mode voltages. A1 amplifies the attenuated signal by 26. The input and output of this amplifier are differential to maximize the ac common-mode rejection. A2 converts the differential voltage from A1 into a single-ended signal and provides further amplification. The gain of this second stage is 32.15. The reference inputs, VREF1 and VREF2, are tied through resistors to the positive input of A2, 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. The gain is 0.5 V/V when they are used to divide the supply. The ratios of Resistors RA, RB, RC, RD, and RF are trimmed to a high level of precision to allow the common-mode rejection ratio to exceed 80 dB. This is accomplished by laser trimming the resistor ratio matching to better than 0.01%. The total gain of 50 is made up of the input attenuation of 1/16.7 multiplied by the first stage gain of 26 and the second stage gain of 32.15. The output stage is Class A with a PNP pull-up transistor and a 300 µA current sink pull-down. AD8205 +IN –IN 250mV GND A1 A2 RA RA RB RB RF RF RD RD RE RF RC RC VOUT RREF RREF VREF1 VREF2 Figure 13. Simplified Schematic D |
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