Datasheet AD8217 (Analog Devices) - 10

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AD8217 THEORY OF OPERATION AMPLIFIER CORE
The AD8217 accurately amplifies the input differential signal, rejecting high voltage common modes ranging from 4.5 V to 80 V. In typical applications, the AD8217 amplifies a small differential input voltage generated by the load current flowing through The main amplifier uses a novel zero-drift architecture, providing a shunt resistor. The AD8217 rejects high common-mode vol- the end user with an extremely stable part over temperature. tages (up to 80 V) and provides a ground-referenced, buffered The offset drift is typically less than ±100 nV/°C. This perfor- output that interfaces with an analog-to-digital converter (ADC). mance leads to optimal accuracy and dynamic range. Figure 24 shows a simplified schematic of the AD8217.
INTERNAL LDO R4
The AD8217 includes an internal LDO, which allows the device
AD8217 I
to power directly from the common-mode voltage at the inputs.
LOAD –IN R1
No additional standalone supply is necessary, provided that the
V2 LOAD SHUNT +IN OUT
common-mode voltage at the +IN pin is at least 4.5 V and up to
V1 R2
80 V. Once the common-mode voltage is above 5.6 V, the LDO
R3 4.5V TO LDO
output reaches its maximum value, that is 5.6 V. This is also the
80V
maximum output voltage range of the AD8217. Because the 24
GND
0 AD8217 output typically interfaces with a converter, the 5.6 V 1- 916 0 maximum output range ensures the ADC input is not damaged Figure 24. Simplified Schematic due to excessive overvoltage. The AD8217 is configured as a difference amplifier. The The input bias current flowing through Pin +IN powers the transfer function is internal LDO and, therefore, doubles as the supply current OUT = (R4/R1) × (V1 − V2) for the AD8217. This current varies depending on the input common-mode voltage. See Figure 8 for additional information. Resistors R4 and R1 are matched to within 0.01% and have values of 1.5 MΩ and 75 kΩ, respectively, meaning an input to output total gain of 20 V/V for the AD8217. Rev. A | 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 CONFIGURATION AND FUNCTION DESCRIPTIONS TYPICAL PERFORMANCE CHARACTERISTICS THEORY OF OPERATION AMPLIFIER CORE INTERNAL LDO APPLICATION NOTES OUTPUT LINEARITY APPLICATIONS INFORMATION HIGH-SIDE CURRENT SENSING MOTOR CONTROL CURRENT SENSING OUTLINE DIMENSIONS ORDERING GUIDE