Datasheet AD8213 (Analog Devices) - 10

link to page 10
AD8213 Data Sheet THEORY OF OPERATION
In typical applications, the AD8213 amplifies a small differential This current (IIN1) is converted back to a voltage via ROUT1. The input voltage generated by the load current flowing through a output buffer amplifier has a gain of 20 V/V, and offers excellent shunt resistor. The AD8213 rejects high common-mode voltages accuracy as the internal gain setting resistors are precision (up to 65 V) and provides a ground referenced, buffered output trimmed to within 0.01% matching. The resulting output that interfaces with an analog-to-digital converter (ADC). voltage is equal to Figure 25 shows a simplified schematic of the AD8213. VOUT1 = (ISHUNT1 × RSHUNT1) × 20 The following explanation refers exclusively to Channel 1 of the Prior to the buffer amplifier, a precision trimmed, 20 kΩ resistor AD8213; however, the same explanation applies to Channel 2. can perform the low-pass filtering of the input signal prior to the A load current flowing through the external shunt resistor amplification stage. By using this resistor, the noise of the input produces a voltage at the input terminals of the AD8213. The signal does not amplify but is rejected, resulting in a more precise input terminals are connected to Amplifier A1 by Resistor R1 (1) output signal that directly interfaces with a converter. A capacitor and Resistor R1 (2). The inverting terminal, which has very high from the CF1 pin to GND, results in a low-pass filter with a input impedance is held to (VCM) − (ISHUNT × RSHUNT), because corner frequency of negligible current flows through Resistor R1 (2). Amplifier A1 1 forces the noninverting input to the same potential. Therefore, f   3dB 2   20000C the current that flows through Resistor R1 FILTER (1), is equal to IIN1 = (ISHUNT1 × RSHUNT1)/R1 (1)
I I SHUNT2 SHUNT1 R R SHUNT2 SHUNT1 IIN2 IIN1 R2 (1) R2 (2) R1 (1) R1 (2) A2 A1 V+ PROPRIETARY PROPRIETARY OFFSET OFFSET CIRCUITRY CIRCUITRY Q2 Q1 20kΩ 20kΩ OUT2 = (ISHUNT2 × RSHUNT2) × 20 OUT1 = (ISHUNT1 × RSHUNT1) × 20 G = +20 ROUT2 ROUT1 G = +20 AD8213
8 02 9-
CF2 GND CF1
63 06 Figure 25. Simplified Schematic Rev. D | Page 10 of 14 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 APPLICATION NOTES OUTPUT LINEARITY LOW-PASS FILTERING APPLICATIONS INFORMATION HIGH-SIDE CURRENT SENSE WITH A LOW-SIDE SWITCH HIGH-SIDE CURRENT SENSING LOW-SIDE CURRENT SENSING BIDIRECTIONAL CURRENT SENSING OUTLINE DIMENSIONS ORDERING GUIDE AUTOMOTIVE PRODUCTS