Datasheet

MAX4194–MAX4197
Micropower, Single-Supply, Rail-to-Rail,
Precision Instrumentation Amplifiers
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Detailed Description
Input Stage
The MAX4194–MAX4197 family of low-power instru-
mentation amplifiers implements a three-amplifier topol-
ogy (Figure 1). The input stage is composed of two
operational amplifiers that together provide a fixed-gain
differential and a unity common-mode gain. The output
stage is a conventional differential amplifier that pro-
vides an overall common-mode rejection of 115dB (G =
+10V/V). The MAX4194’s gain can be externally set
between +1V/V and +10,000V/V (Table 1). The
MAX4195/MAX4196/MAX4197 have on-chip gain-set-
ting resistors (Figure 2), and their gains are fixed at
+1V/V, +10V/V, and +100V/V, respectively.
Input Voltage Range
and Detailed Operation
The common-mode input range for all of these ampli-
fiers is V
EE
- 0.2V to V
CC
- 1.1V. Ideally, the instrumen-
tation amplifier (Figure 3) responds only to a differential
voltage applied to its inputs, IN+ and IN-. If both inputs
are at the same voltage, the output is V
REF
. A differen-
tial voltage at IN+ (V
IN+
) and IN- (V
IN-
) develops an
identical voltage across the gain-setting resistor, caus-
ing a current (I
G
) to flow. This current also flows
through the feedback resistors of the two input ampli-
fiers A1 and A2, generating a differential voltage of:
V
OUT2
- V
OUT1
= I
G
· (R
1
+ R
G
+ R
1
)
where V
OUT1
and V
OUT2
are the output voltages of A1
and A2, R
G
is the gain-setting resistor (internal or exter-
nal to the part), and R1 is the feedback resistor of the
input amplifiers.
I
G
is determined by the following equation:
I
G
= (V
IN+
- V
IN-
) / R
G
The output voltage (V
OUT
) for the instrumentation
amplifier is expressed in the following equation:
V
OUT
= (V
IN+
- V
IN-
) · [(2 · R1) / R
G
] + 1
The common-mode input range is a function of the
amplifier’s output voltage and the supply voltage. With
a power supply of V
CC
, the largest output signal swing
can be obtained with REF tied to V
CC
/2. This results in
an output voltage swing of ±V
CC
/2. An output voltage
swing less than full-scale increases the common-mode
input range.
R
1
*
R
1
*
R
2
*
R
2
*
R
2
*
R
2
*
V
IN-
REF
OUT
R
G
**
I
G
I
G
V
IN+
V
OUT2
V
OUT1
V
OUT2
- V
OUT1
V
IN+
- V
IN-
A1
A2
A3
V
OUT
= (V
IN+
- V
IN-
) ·
(
1 +
)
2R
1
R
G
* R
1
= R
2
= 25kΩ
** R
G
= INTERNAL TO MAX4195/MAX4196/MAX4197
R
G
= EXTERNAL TO MAX4194
Figure 3. Instrumentation Amplifier Configuration
25kΩ
25kΩ
25kΩ
25kΩ
25kΩ
25kΩ
IN-
RG-
OUT
REF
V
EE
V
CC
IN+
RG+
MAX4194
Figure 1. MAX4194 Simplified Block Diagram
25kΩ
25kΩ
25kΩ
25kΩ
25kΩ
25kΩ
IN-
SHDN
OUT
REF
V
CC
V
EE
FB
R
G
IN+
MAX4195
MAX4196
MAX4197
Figure 2. MAX4195/MAX4196/MAX4197 Simplified Block
Diagram