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-
rail output voltage, V
OUT
.
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
C
FILTER
is used to filter out correction noise.
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
I – I
R1
M2
VINP
M3 M4
M1
R1
(V
INP
– V
INN
)
I
R1
=
R1
2I
2I
VINN
V
BIAS
M5
M6
I – I
R1
I + I
R1
2I
R1
C
FILTER
R2
R3
V
REF
= 0.9V
A1
V
INP
– V
INN
R1
2R2
V
OUT
= V
REF
EXTERNAL
+
06745-023
CC
Figure 43. Simplified Schematic of the Instrumentation Amplifier Within the AD8290
