Datasheet AD629 (Analog Devices) - 10

AD629 THEORY OF OPERATION
The AD629 is a unity gain, differential-to-single-ended To reduce output drift, the op amp uses super beta transistors amplifier (diff amp) that can reject extremely high common- in its input stage. The input offset current and its associated mode signals (in excess of 270 V with 15 V supplies). It consists temperature coefficient contribute no appreciable output of an operational amplifier (op amp) and a resistor network. voltage offset or drift, which has the added benefit of reducing To achieve high common-mode voltage range, an internal voltage noise because the corner where 1/f noise becomes resistor divider (Pin 3 or Pin 5) attenuates the noninverting dominant is below 5 Hz. To reduce the dependence of gain signal by a factor of 20. Other internal resistors (Pin 1, Pin 2, accuracy on the op amp, the open-loop voltage gain of the op and the feedback resistor) restore the gain to provide a differential amp exceeds 20 million, and the PSRR exceeds 140 dB. gain of unity. The complete transfer function equals
21.1kΩ 380kΩ REF(–) 1 8 NC
VOUT = V (+IN) − V (−IN)
380kΩ –IN 2 7 +VS
Laser wafer trimming provides resistor matching so that
380kΩ +IN 3 6 OUTPUT
common-mode signals are rejected while differential input
20kΩ
signals are amplified.
–V 4 5 S REF(+) AD629
01 0 3-
NC = NO CONNECT
078 0 Figure 31. Functional Block Diagram Rev. C | 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 APPLICATIONS BASIC CONNECTIONS SINGLE-SUPPLY OPERATION SYSTEM-LEVEL DECOUPLING AND GROUNDING USING A LARGE SENSE RESISTOR OUTPUT FILTERING OUTPUT CURRENT AND BUFFERING A GAIN OF 19 DIFFERENTIAL AMPLIFIER ERROR BUDGET ANALYSIS EXAMPLE 1 ERROR BUDGET ANALYSIS EXAMPLE 2 OUTLINE DIMENSIONS ORDERING GUIDE