AD8293G80/AD8293G160

Rev. 0 | Page 10 of 16

THEORY OF OPERATION

The AD8293G80/AD8293G160 are precision current-mode

correction instrumentation amplifiers capable of single-supply

operation. The current-mode correction topology results in

excellent accuracy. Figure 18 shows a simplified diagram

illustrating the basic operation of the AD8293G80/AD8293G160

(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

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.

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 AD8293G80/AD8293G160

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

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

AD8293G80/AD8293G160. This allows the AD8293G80/

AD8293G160 to have lower noise near dc than standard low

noise instrumentation amplifiers.

II

M2

M3

M4

M1

R1

R1

2I

2I

M5

M6

C2

R2

R3

A1

R1

2R2

EXTERNAL

+

C3

Figure 18. Simplified Schematic