Datasheet

Detailed Description
The MAX4208/MAX4209 family of instrumentation ampli-
fiers implements a spread-spectrum, autozeroing tech-
nique that minimizes the input offset error, drift over
time and temperature, and the effect of 1/f noise. Unlike
the traditional three-op amp instrumentation amplifier,
this technique allows true ground-sensing capability
combined with a low input bias current and increased
common-mode rejection.
The differential input signal is converted to a current by
an input transconductance stage. An output transconduc-
tance stage converts a portion of the output voltage (equal
to the output voltage divided by the gain) into another pre-
cision current. These two currents are subtracted and the
result is fed to a loop amplifier with sufficient gain to mini-
mize errors (Figures 1 and 2). The MAX4209 has a facto-
ry-trimmed gain of 100V/V. The MAX4208 has an adjust-
able gain, set with an external pair of resistors between
OUT, FB, and REF (Figure 1). The MAX4208/MAX4209
have an output reference input (REF) that is connected to
an external reference for bipolar operation of the device.
For single-supply operation, the range for V
REF
is 0V to
(V
DD
- 1.3V). Although full output-swing capability and
maximum symmetrical dynamic range is obtained at REF
= V
DD
/2, the optimal V
REF
setting depends on the supply
voltage and output-voltage swing needed by the appli-
cation. The maximum recommended differential input
voltage is ±100mV. Linearity and accuracy are degraded
above that level. The MAX4208/MAX4209 operate with
single 2.85V to 5.5V supply voltages or dual ±1.425V to
±2.75V supplies.
The MAX4208/MAX4209 have a shutdown feature to
reduce the supply current to 1.4μA (typ) when REFIN/
MODE is connected to V
DD
.
REF, REFIN/MODE, and Internal REFIN
Buffer of the MAX4208/MAX4209
In a single-supply system, bipolar operation of an instru-
mentation amplifier requires the application of a voltage
reference (REF) to set the output voltage level when a
zero differential voltage is applied to the input. The output
swing is around this reference level, which is usually set
to half of the supply voltage for the largest swing and
dynamic range.
In many instrumentation amplifiers, the gain-setting resis-
tors as well as the R
L
are connected between OUT and
REF. OUT can sink and source current but the need for
REF to sink and source current is often overlooked and
can lead to significant errors. Therefore, the MAX4208/
MAX4209 include a REFIN buffer, an internal, precision
unity-gain buffer on-chip to sink and source the currents
needed at REF without loading the reference voltage
supplied at REFIN/MODE.
Figure 2. MAX4209 Functional DiagramFigure 1. MAX4208 Functional Diagram
MAX4208
g
m
g
m
AMP
SHDN
+1
OUT
FB
REF
REFIN/MODE
R2
R1
IN-
IN+
V
DD
V
SS
G = 1 +
R2
R1
MAX4209
g
m
g
m
AMP
SHDN
+1
OUT
FB
REF
REFIN/MODE
R2
R1
IN-
IN+
V
DD
V
SS
G = 1 +
R2
R1
MAX4208/MAX4209 Ultra-Low Offset/Drift, Precision
Instrumentation Ampliers with REF Buffer
www.maximintegrated.com
Maxim Integrated │ 11