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
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 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 V
REF
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.
