Datasheet AD8293G80, AD8293G160 (Analog Devices) - 10

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AD8293G80/AD8293G160 THEORY OF OPERATION
The AD8293G80/AD8293G160 are precision current-mode
HIGH PSR AND CMR
correction instrumentation amplifiers capable of single-supply Common-mode rejection and power supply rejection indicate operation. The current-mode correction topology results in the amount that the offset voltage of an amplifier changes when excellent accuracy. Figure 18 shows a simplified diagram its common-mode input voltage or power supply voltage changes. illustrating the basic operation of the AD8293G80/AD8293G160 The autocorrection architecture of the AD8293G80/AD8293G160 (without correction). The circuit consists of a voltage-to-current continuously corrects for offset errors, including those induced amplifier (M1 to M6), followed by a current-to-voltage amplifier by changes in input or supply voltage, resulting in exceptional (R2 and A1). Application of a differential input voltage forces a rejection performance. The continuous autocorrection provides current through External Resistor R1, resulting in conversion of great CMR and PSR performances over the entire operating the input voltage to a signal current. Transistor M3 to Transistor temperature range (−40°C to +85°C). M6 transfer twice this signal current to the inverting input of the op amp A1. Amplifier A1 and External Resistor R2 form The parasitic resistance in series with R2 does not degrade CMR, a current-to-voltage converter to produce a rail-to-rail output but causes a small gain error and a very small offset error. voltage at V Therefore, an external buffer amplifier is not required to drive OUT. VREF to maintain excellent CMR performance. This helps reduce Op amp A1 is a high precision auto-zero amplifier. This amplifier system costs over conventional instrumentation amplifiers. preserves the performance of the autocorrecting, current-mode amplifier topology while offering the user a true voltage-in,
1/f NOISE CORRECTION
voltage-out instrumentation amplifier. Offset errors are corrected Flicker noise, also known as 1/f noise, is noise inherent in the internally. physics of semiconductor devices and decreases 10 dB per decade. An external reference voltage is applied to the noninverting The 1/f corner frequency of an amplifier is the frequency at which input of A1 to set the output reference level. External Capacitor the flicker noise is equal to the broadband noise of the amplifier. At C2 is used to filter out correction noise. lower frequencies, flicker noise dominates, causing large errors in low frequency or dc applications. Flicker noise is seen effectively as a slowly varying offset error, which is reduced by the autocorrection topology of the AD8293G80/AD8293G160. This allows the AD8293G80/ AD8293G160 to have lower noise near dc than standard low noise instrumentation amplifiers.
VCC C2 2R2 I I M5 M6 R2 V V
+
INP – VINN OUT = VREF R1 R1 I – IR1 I – I 2IR1 R1 R3 I + I (V R1 INP – VINN) IR1 = A1 C3 R1 VBIAS V V REF INP VINN M3 M4
0
M1 M2
02 1- 45 07
2I 2I EXTERNAL
Figure 18. Simplified Schematic Rev. 0 | Page 10 of 16 Document Outline FEATURES APPLICATIONS GENERAL DESCRIPTION FUNCTIONAL BLOCK DIAGRAM TABLE OF CONTENTS REVISION HISTORY SPECIFICATIONS ELECTRICAL CHARACTERISTICS ABSOLUTE MAXIMUM RATINGS THERMAL RESISTANCE ESD CAUTION PIN CONFIGURATION AND FUNCTION DESCRIPTIONS TYPICAL PERFORMANCE CHARACTERISTICS THEORY OF OPERATION HIGH PSR AND CMR 1/f NOISE CORRECTION APPLICATIONS INFORMATION OVERVIEW REFERENCE CONNECTION OUTPUT FILTERING CLOCK FEEDTHROUGH POWER SUPPLY BYPASSING INPUT OVERVOLTAGE PROTECTION OUTLINE DIMENSIONS ORDERING GUIDE