Datasheet

INA326, INA327
12
SBOS222D
www.ti.com
The INA326 uses a new, unique internal circuit topology
that provides true rail-to-rail input. Unlike other instrumen-
tation amplifiers, it can linearly process inputs up to 20mV
below the negative power-supply rail, and 100mV above
the positive power-supply rail. Conventional instrumenta-
tion amplifier circuits cannot deliver such performance,
even if rail-to-rail op amps are used.
The ability to reject common-mode signals is derived in
most instrumentation amplifiers through a combination of
amplifier CMR and accurately matched resistor ratios.
The INA326 converts the input voltage to a current.
Current-mode signal processing provides rejection of com-
mon-mode input voltage and power-supply variation with-
out accurately matched resistors.
A simplified diagram shows the basic circuit function. The
differential input voltage, (V
IN+
) – (V
IN–
) is applied across
R
1
. The signal-generated current through R
1
comes from
A1 and A2’s output stages. A2 combines the current in R
1
with a mirrored replica of the current from A1. The result-
ing current in A2’s output and associated current mirror is
two times the current in R
1
. This current flows in (or out)
of pin 5 into R
2
. The resulting gain equation is:
G
R
R
= 2
2
1
Amplifiers A1, A2, and their associated mirrors are pow-
ered from internal charge-pumps that provide voltage
supplies that are beyond the positive and negative supply
rails. As a result, the voltage developed on R
2
can actually
swing 20mV
below the negative power-supply rail, and
100mV
above the positive supply rail. A3 provides a
buffered output of the voltage on R
2
. A3’s input stage is
also operated from the charge-pumped power supplies for
true rail-to-rail operation.
FIGURE 5. Simplified Circuit Diagram.
INSIDE THE INA326
A1
V+ V−
Current Mirror
Current Mirror
I
R1
I
R1
I
R1
R
1
R
2
C
2
V
O
V
IN−
V
IN+
I
R1
2I
R1
2I
R1
2I
R1
2I
R1
2I
R1
A3
A2
IA
COMMON
0.1µF
Current Mirror
Current Mirror
74
6
5
3
8
1
2
INA326