Datasheet

Applications Information
Setting the Gain (MAX4208)
Connect a resistive divider from OUT to REF with the cen-
ter tap connected to FB to set the gain for the MAX4208
(see the Typical Application Circuit). Calculate the gain
using the following formula:
R2
GAIN 1
R1

= +


Choose a value for R1 ≤ 1kΩ. Resistor accuracy ratio
directly affects gain accuracy. Resistor sum less than
10kΩ should not be used because their loading can
slightly affect output accuracy.
Input Common Mode vs.
Input Differential-Voltage Range
Traditional three-op amp instrumentation amplifiers have
a defined relationship between the maximum input dif-
ferential voltage and maximum input common-mode
voltage that arises from saturation of intermediate ampli-
fier stages. This correlation is frequently represented
as a hexagon graph of input common-mode voltage vs.
output voltage for the instrumentation amplifier shown in
Figure 3. Application limitations hidden in this graph are:
● The input common-mode voltage range does not
include the negative supply rail, and so no amplifica-
tion is possible for inputs near ground for single-supply
applications.
● Input differential voltages can be amplified with maxi-
mum gain only over a limited range of input common-
mode voltages (i.e., range of y-axis for max range of
x-axis is limited).
● If large amplitude common-mode voltages need to
be rejected, differential voltages cannot be amplified
with a maximum gain possible (i.e., range of x-axis
for a maximum range of y-axis is limited). As a conse-
quence, a secondary high-gain amplifier is required to
follow the front-end instrumentation amplifier.
The indirect current-feedback architecture of the
MAX4208/MAX4209 instrumentation amplifiers do not
suffer from any of these drawbacks. Figure 4 shows the
input common-mode voltage vs. output voltage graph of
indirect current-feedback architecture.
In contrast to three-op amp instrumentation amplifiers, the
MAX4208/MAX4209 features:
● The input common-mode voltage range, which
includes the negative supply rail and is ideal for single-
supply applications.
● Input differential voltages that can be amplified with
maximum gain over the entire range of input common-
mode voltages.
● Large common-mode voltages that can be rejected at
the same time differential voltages are amplified with
maximum gain, and therefore, no secondary ampli-
fier is required to follow the front-end instrumentation
amplifier.
Gain Error Drift Over Temperature
Adjustable gain instrumentation amplifiers typically use a
single external resistor to set the gain. However, due to
differences in temperature drift characteristics between
the internal and external resistors, this leads to large
gain-accuracy drift over temperature. The MAX4208 is an
adjustable gain instrumentation amplifier that uses two
external resistors to set its gain. Since both resistors are
external to the device, layout and temperature coefficient
matching of these parts deliver a significantly more stable
gain over operating temperatures.
The fixed gain, MAX4209H has both internal resistors for
excellent matching and tracking.
Use of External Capacitor C
FB
for Noise Reduction
Zero-drift chopper amplifiers include circuitry that continu-
ously compensates the input offset voltage to deliver pre-
cision and ultra-low temperature drift characteristics. This
self-correction circuitry causes a small additional noise
contribution at its operating frequency (a psuedorandom
clock around 45kHz for MAX4208/MAX4209). For high-
bit resolution ADCs, external filtering can significantly
attenuate this additional noise. Simply adding a feedback
capacitor (C
FB
) between OUT and FB reduces high-
frequency gain, while retaining the excellent precision
DC characteristics. Recommended values for C
FB
are
between 1nF and 10nF. Additional anti-aliasing filtering
at the output can further reduce this autocorrection noise.
Capacitive-Load Stability
The MAX4208/MAX4209 are capable of driving capacitive
loads up to 200pF. Applications needing higher capacitive
drive capability may use an isolation resistor between
OUT and the load to reduce ringing on the output signal.
However, this reduces the gain accuracy due to the volt-
age drop across the isolation resistor.
MAX4208/MAX4209 Ultra-Low Offset/Drift, Precision
Instrumentation Ampliers with REF Buffer
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