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AD8553ARMZ-R2 데이터시트(PDF) 11 Page - Analog Devices |
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AD8553ARMZ-R2 데이터시트(HTML) 11 Page - Analog Devices |
11 / 20 page AD8553 Rev. A | Page 11 of 20 THEORY OF OPERATION The AD8553 is a precision current-mode correction instrumentation amplifier capable of single-supply operation. The current-mode correction topology results in excellent accuracy without the need for trimmed resistors on the die. Figure 30 shows a simplified diagram illustrating the basic operation of the AD8553 (without correction). The circuit consists of a voltage-to-current amplifier (M1 to M6), followed by a current-to-voltage amplifier (R2 and A1). Application of a differential input voltage forces a current through External Resistor R1, resulting in conversion of the input voltage to a signal current. Transistor M3 to Transistor M6 transfer twice this signal current to the inverting input of the op amp A1. Amplifier A1 and External Resistor R2 form a current-to- voltage converter to produce a rail-to-rail output voltage at VOUT. Op amp A1 is a high precision auto-zero amplifier. This amplifier preserves the performance of the autocorrecting, current-mode amplifier topology while offering the user a true voltage-in, voltage-out instrumentation amplifier. Offset errors are corrected internally. An external reference voltage is applied to the noninverting input of A1 to set the output reference level. External Capacitor C2 is used to filter out correction noise. The pinout of the AD8553 allows the user to access the signal current from the output of the voltage-to-current converter (Pin 5). The user can choose to use the AD8553 as a current- output device instead of a voltage-output device. See Figure 35 for circuit connections. HIGH PSR AND CMR Common-mode rejection and power supply rejection indicate the amount that the offset voltage of an amplifier changes when its common-mode input voltage or power supply voltage changes. The autocorrection architecture of the AD8553 continuously corrects for offset errors, including those induced by changes in input or supply voltage, resulting in exceptional rejection performance. The continuous autocorrection provides great CMR and PSR performances over the entire operating temperature range (−40°C to +85°C). The parasitic resistance in series with R2 does not degrade CMR but causes a small gain error and a very small offset error. Therefore, an external buffer amplifier is not required to drive the VREF pin to maintain excellent CMR performance. This helps reduce system costs over conventional instrumentation amplifiers. 1/f NOISE CORRECTION Flicker noise, also known as 1/f noise, is noise inherent in the physics of semiconductor devices and decreases 10 dB per decade. The 1/f corner frequency of an amplifier is the frequency at which the flicker noise is equal to the broadband noise of the amplifier. At 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 AD8553. This allows the AD8553 to have lower noise near dc than standard low noise instrumentation amplifiers. |
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