Datasheet AD8218 (Analog Devices) - 10

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AD8218 Data Sheet THEORY OF OPERATION AMPLIFIER CORE
The AD8218 is configured as a difference amplifier. The transfer function is In typical applications, the AD8218 amplifies a small differential input voltage generated by the load current flowing through OUT = ((R4/R1) × (V1 − V2)) + VREF a shunt resistor. The AD8218 rejects high common-mode vol- Resistors R4 and R1 are matched to within 0.01% and have tages (up to 80 V) and provides a ground-referenced, buffered values of 1.5 MΩ and 75 kΩ, respectively, meaning an input- output. Figure 28 shows a simplified schematic of the AD8218. to-output total gain of 20 V/V for the AD8218. The difference between V1 and V2 is the voltage across the shunt resistor, or
5V CF
VIN. Therefore, the input-to-output transfer function of the
ILOAD
AD8218 is
V GND S ICHARGE AD8218 R4
OUT (V) = (20 × VIN) + VREF The AD8218 accurately amplifies the input differential signal,
–IN R1 V
rejecting high voltage common modes ranging from 4 V to 80 V.
2 LOAD SHUNT +IN OUT V1
The main amplifier uses a novel zero-drift architecture, providing
R2 4V LDO R3
the end user with breakthrough temperature stability. The
TO 80V ENB
offset drift is typically less than ±100 nV/°C. This performance leads to optimal accuracy and dynamic range.
REF GND VREF OUTPUT CLAMPING
027 09592- After the input common-mode voltage in the application is Figure 28. Simplified Schematic above 5.2 V, the internal LDO output of the AD8218 also reaches its maximum value of 5.2 V, which is the maximum output range of the AD8218. Because in typical applications the output interfaces with a converter, clamping the AD8218 output voltage to 5.2 V ensures that the ADC input is not damaged due to excessive overvoltage. Rev. B | Page 10 of 16 Document Outline Features Applications Functional Block Diagram General Description Table of Contents Revision History Specifications Absolute Maximum Ratings ESD Caution Pin Configurations and Function Descriptions Typical Performance Characteristics Theory of Operation Amplifier Core Output Clamping Application Notes Supply (VS) Connections Enable Pin (ENB) Operation Applications Information Unidirectional High-Side Current Sensing Bidirectional High-Side Current Sensing Motor Control Current Sensing Outline Dimensions Ordering Guide