AD8290

Rev. B | Page 14 of 20

THEORY OF OPERATION

AMPLIFIER

The amplifier of the AD8290 is a precision current-mode

correction instrumentation amplifier. It is internally set to a

fixed gain of 50. The current-mode correction topology results

in excellent accuracy.

Figure 43 shows a simplified diagram illustrating the basic

operation of the instrumentation amplifier within the AD8290

(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 R1, resulting in a conversion of the input

voltage to a signal current. Transistors M3 to M6 transfer twice

the signal current to the inverting input of the op amp, A1. A1

and R2 form a current-to-voltage converter to produce a rail-to-

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 internal 0.9 V reference voltage is applied to the noninverting

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

HIGH POWER SUPPLY REJECTION (PSR) AND

COMMON-MODE REJECTION (CMR)

PSR and CMR 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

AD8290 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).

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 appears as a slowly varying offset error that is

reduced by the autocorrection topology of the AD8290, allowing

the AD8290 to have lower noise near dc than standard low

noise instrumentation amplifiers.

I

I

M2

VINP

M3

M4

M1

R1

R1

2I

2I

VINN

M5

M6

R2

R3

A1

R1

2R2

EXTERNAL

+

Figure 43. Simplified Schematic of the Instrumentation Amplifier Within the AD8290