MAX4208/MAX4209
Ultra-Low Offset/Drift, Precision
Instrumentation Amplifiers with REF Buffer
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In a conventional instrumentation amplifier, a simple
method to apply a reference voltage is the use of a volt-
age-divider to set the REF level (often halfway between
ground and V
DD
). The voltage-divider should be made
of higher value resistors to minimize current consump-
tion, but the sinking and sourcing current from the load
and gain-setting resistors create a significant common-
mode signal at the divider midpoint. The MAX4208/
MAX4209 precision REFIN buffer essentially eliminates
the error voltage at REF.
The REFIN buffer is a unity-gain op amp that has a guar-
anteed V
OS
of less than 40µV with a CMOS input bias
current of only 1pA, to allow setting REFIN with a simple
resistive divider with minimum errors.
REFIN/MODE is a triple function input (see Table 1). To
use the internal REFIN buffer, connect REFIN/MODE to
an external reference or a simple resistive divider at
any voltage between (V
SS
+ 0.2V) and (V
DD
- 1.3V).
These voltages represent the minimum and maximum
for the REFIN buffer’s input common-mode range (see
the Electrical Characteristics table). To use ground at
REF or to use an external low-impedance reference
directly at REF without the internal REFIN buffer, con-
nect REFIN/MODE to V
SS
. This disables the REFIN
buffer, dropping the I
DD
to 750µA and puts the REFIN
buffer output in a high-impedance state to allow exter-
nal direct drive of REF. To put the MAX4208/MAX4209
into shutdown and reduce the supply current to less
than 5µA, drive REFIN/MODE to V
DD
.
Note: When driving REF directly, REFIN/MODE must be
at V
SS
and shutdown mode is NOT available.
Input Differential Signal Range
The MAX4208/MAX4209 feature a proprietary input
structure optimized for small differential signals of up to
±100mV. The output of the MAX4208/MAX4209 allows
for bipolar input signals. The output voltage is equal to
the voltage at REF for zero differential input. The gain
accuracy of these devices is laser trimmed to better
than 0.1% (typ).
Output Swing
The MAX4208/MAX4209 are designed specifically for
small input signals (±100mV) from sensors, strain
gauges, etc. These instrumentation amplifiers are
capable of rail-to-rail output-voltage swings; however,
depending on the selected gain and REF level, the rail-
to-rail output swing may not be required or desired.
For example, consider single-supply operation of the
MAX4208 in a unity-gain configuration with REF con-
nected to a voltage at half of the supply voltage (V
DD
/
2). In this case, the output-voltage swing would be
±100mV around the REF level and would not need to
reach either rail.
Another example is the MAX4209H (gain internally set
to 100) also operating with a single-supply voltage and
REF set externally to ground (V
SS
). REFIN/MODE must
also be connected to ground (V
SS
). In this case, an
input voltage of 0 to 10mV differential would ideally
drive an output-voltage swing of 0 to 1V. However, the
output swing can only get to within 40mV of ground
(V
SS
) (see the V
OL
specifications in the Electrical
Characteristics table). It is recommended that for best
accuracy and linearity, the lowest differential input volt-
age for unipolar operation is usually picked to be a
nonzero value (e.g., 0.5mV or more).
Another remedy is to use REFIN/MODE of 250mV (see
the REFIN/MODE Buffered Reference Input Range in the
Electrical Characteristics table), which causes a 0 to
100mV input to start OUT at 250mV and swing to 1.25V,
to prevent the output from going into its bottom nonlinear
range. An ADC with differential input can be connected
between OUT and REF to record the true 0 to 1V swing.
Devices with higher gain and bipolar output swing can
be configured to approach either rail for maximum
dynamic range. However, as the output approaches with-
in V
OL
or V
OH
of the supply voltages, the linearity and
accuracy degrades, especially under heavy loading.
Table 1. REFIN/MODE Pin Functions
The internal REF buffer is activated. REF MUST NOT be fed by any external source. The voltage
of the internal REF buffer).
The internal REF buffer is OFF with its output in a high-impedance state to allow direct drive of
REF (or connection to ground). REF must be directly connected to an external voltage reference
capable of sinking and sourcing the load current.
