AD8563

Preliminary Technical Data

Rev. PrA | Page 6 of 15

THEORY OF OPERATION

The AD8563 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 2 shows a simplified diagram illustrating the basic

operation of the AD8563 (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

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 non-inverting

input of A1 to set the output reference level. External Capacitor

C2 filters out correction noise.

The pin out of the AD8563 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 AD8563 as a current-

output device instead of a voltage-output device. See Figure 7

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 auto-correction architecture of the AD8563 continuously

corrects for offset errors, including those induced by changes in

input or supply voltage, resulting in exceptional rejection

performance. The continuous auto-correction provides great

CMR and PSR performances over the entire operating

temperature range (−40°C to +125°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

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 effectively visible as a slowly varying offset error,

which the auto-correction topology of the AD8563 reduces. This

allows the AD8563 to have lower noise near dc than standard

low noise instrumentation amplifiers.