Datasheet AD8205 (Analog Devices) - 8

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AD8205 Data Sheet THEORY OF OPERATION
The AD8205 is a single-supply difference amplifier that uses a By attenuating the voltages at Pin 1 and Pin 8, the A1 amplifier unique architecture to accurately amplify small differential inputs are held within the power supply range, even if Pin 1 and current shunt voltages in the presence of rapidly changing Pin 8 exceed the supply or fal below common (ground). A common-mode voltages. reference voltage of 250 mV biases the attenuator above ground. In typical applications, the AD8205 is used to measure current This al ows the amplifier to operate in the presence of negative by amplifying the voltage across a current shunt placed across common-mode voltages. the inputs. The input network also attenuates normal (differential) mode The gain of the AD8205 is 50 V/V, with an accuracy of 1.2%. voltages. A1 amplifies the attenuated signal by 26. The input This accuracy for the Y and W Grade is guaranteed over the and output of this amplifier are differential to maximize the ac operating temperature range of −40°C to +125°C. The H Grade common-mode rejection. version of the AD8205 is specified for operation from −40°C to A2 converts the differential voltage from A1 into a single-ended +150°C. signal and provides further amplification. The gain of this The input offset is less than 2 mV referred to the input at 25°C, second stage is 32.15. and 4.5 mV maximum referred to the input over the ful The reference inputs, VREF1 and VREF2, are tied through resistors operating temperature range for the packaged part. to the positive input of A2, which al ows the output offset to be The AD8205 operates with a single supply from 4.5 V to adjusted anywhere in the output operating range. The gain is 10 V (absolute maximum = 12.5 V). The supply current is 1 V/V from the reference pins to the output when the reference less than 2 mA. pins are used in parallel. The gain is 0.5 V/V when they are used to divide the supply. High accuracy trimming of the internal resistors allows the AD8205 to have a common-mode rejection ratio better than The ratios of Resistors RA, RB, RC, RD, and RF are trimmed to a 78 dB from dc to 20 kHz. high level of precision to al ow the common-mode rejection ratio to exceed 80 dB. This is accomplished by laser trimming The output offset can be adjusted from 0.05 V to 4.8 V (V+ = 5 V) the resistor ratio matching to better than 0.01%. for unipolar and bipolar operation. The total gain of 50 is made up of the input attenuation of The AD8205 consists of two amplifiers (A1 and A2), a resistor 1/16.7 multiplied by the first stage gain of 26 and the second network, small voltage reference, and a bias circuit (not shown), stage gain of 32.15. see Figure 13. The output stage is Class A with a PNP pul -up transistor and a The set of input attenuators preceding A1 consist of RA, RB, and 300 µA current sink pull-down. RC, which reduce the common-mode voltage to match the input
–IN +IN
voltage range of A1. The two attenuators form a balanced bridge network. When the bridge is balanced, the differential voltage
RA RA
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
A1
resistance of RA, RB, and RC is approximately 200 kΩ ± 20%.
RB RB RF RF RD RD A2 R V C RC OUT V AD8205 R REF1 E RF 250mV RREF RREF GND VREF2
04315-0-012 Figure 13. Simplified Schematic Rev. D | Page 8 of 12 Document Outline Features Excellent AC and DC Performance Applications Functional Block Diagram General Description Revision History Specifications Absolute Maximum Ratings ESD Caution Pin Configuration and Function Descriptions Typical Performance Characteristics Theory of Operation Output Offset Adjustment Unidirectional Operation Ground Referenced Output V+ Referenced Output Bidirectional Operation External Reference Output Splitting the Supply Splitting an External Reference Applications Information High-Side Current Sense with a Low-Side Switch High-Side Current Sense with a High-Side Switch Outline Dimensions Ordering Guide Automotive Products