Datasheet

Power-Supply Bypass and Layout
Good layout technique optimizes performance by decreas-
ing the amount of stray capacitance at the instrumenta-
tion amplifier’s gain-setting pins (OUT, FB, and REF).
Excess capacitance produces peaking in the amplifier’s
frequency response. To decrease stray capacitance,
minimize trace lengths by placing external components
as close as possible to the instrumentation amplifier.
Unshielded long traces at the inputs of the instrumenta-
tion amplifier degrade the CMRR and pick-up noise. This
produces inaccurate output in highgain configurations.
Use shielded or coax cables to connect the inputs of the
instrumentation amplifier. Since the MAX4208/MAX4209
feature ultra-low input offset voltage, board leakage
and thermocouple effects can easily introduce errors in
the input offset voltage readings when used with high-
impedance signal sources. Minimize board leakage cur-
rent and thermocouple effects by thoroughly cleaning the
board and placing the matching components very close
to each other and with appropriate orientation. For best
performance, bypass each power supply to ground with a
separate 0.1μF capacitor.
For noisy digital environments, the use of multilayer PCB
with separate ground and power-supply planes is recom-
mended. Keep digital signals far away from the sensitive
analog inputs.
Refer to the MAX4208 or MAX4209 Evaluation Kit data
sheets for good layout examples.
Low-Side Current-Sense Amplier
The use of indirect current-feedback architecture makes
the MAX4208/MAX4209 ideal for low-side current-sens-
ing applications, i.e., where the current in the circuit
ground needs to be measured by means of a small sense
resistor. In these situations, the input common-mode
voltage is allowed to be at or even slightly below ground
(V
SS
- 0.1V).
If the currents to be measured are bidirectional, connect
REFIN/MODE to V
DD
/2 to get full dynamic range for each
direction. If the currents to be measured are unidirec-
tional, both REFIN/MODE and REF can be tied to GND.
However, V
OL
limitations can limit low-current measure-
ment. If currents need to be measured down to 0A, bias
REFIN/MODE to a voltage above 0.2V to activate the
internal buffer and to stay above amplifier V
OL
, and mea-
sure both OUT and REF with a differential input ADC.
Low-Voltage, High-Side
Current-Sense Amplier
Power management is a critical area in high-performance
portable devices such as notebook computers. Modern
digital processors and ASICs are using smaller transis-
tor geometries to increase speed, reduce size, and also
lower their operating core voltages (typically 0.9V to
1.25V). The MAX4208/MAX4209 instrumentation ampli-
fiers can be used as a nearly zero voltage-drop, current-
sense amplifier (see Figure 5).
Figure 3. Limited Common Mode vs. Output Voltage of a
Three Op-Amp INA
Figure 4. Input Common Mode vs. Output Voltage of
MAX4208/MAX4209 Includes 0V (GND)
V
CM
V
CC
V
CM-MAX
3/4 V
CC
1/2 V
CC
1/4 V
CC
0
V
CC
/2
V
CC
V
OUT
( = GAIN x V
DIFF
+ V
REF
)
V
REF
= 1/2 V
CC
CLASSIC THREE OP-AMP INA
V
CM
V
DD
V
CM-MAX
0
V
DD
/2
V
DD
V
OUT
( = GAIN x V
DIFF
+ V
REF
)
V
REF
= 1/2 V
DD
MAX4208/MAX4209
MAX4208/MAX4209 Ultra-Low Offset/Drift, Precision
Instrumentation Ampliers with REF Buffer
www.maximintegrated.com
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