Datasheet

MAX9922/MAX9923
Ultra-Precision, High-Side
Current-Sense Amplifiers
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Shutdown
The MAX9922/MAX9923 feature a logic shutdown input
to reduce the supply current to less than 1µA. Drive
SHDN high for normal operation. Drive SHDN low to
place the device in shutdown mode. In shutdown
mode, the current drawn from both the V
DD
input and
the current-sense amplifier inputs (RSB, RS+, and RS-)
is less than 1µA each.
External Reference
The MAX9922/MAX9923 are capable of both unidirec-
tional and bidirectional operation. For unidirectional
current-sense applications, connect the REF input to
GND. For bidirectional, connect REF to a reference.
This sets bidirectional current sense with V
OUT
= V
REF
for V
SENSE
= 0mV. Positive V
SENSE
causes OUT to
swing toward the positive supply, while negative
V
SENSE
causes OUT to swing toward GND. This feature
allows the output voltage to measure both charge and
discharge currents. Use V
REF
= V
DD
/2 for maximum
dynamic range.
In bidirectional operation, the external voltage applied
to V
REF
has to be able to supply the current in the feed-
back network between OUT, FB, and REF. This current
is simply the input sense voltage divided by the resis-
tance between FB and REF (1kΩ typical for MAX9923).
Furthermore, ensure the external voltage source sup-
plied to REF has a low source resistance to prevent
gain errors (e.g., use a stand-alone reference voltage
or an op amp to buffer a high-value resistor string.) See
the Typical Operating Circuits .
Input Differential Signal Range
The MAX9922/MAX9923 feature a proprietary input
structure optimized for small differential signals as low
as 10mV full scale for high efficiency with lowest power
dissipation in the sense resistor, or +100mV full scale for
high dynamic range. The output of the MAX9922/
MAX9923 allows for bipolar input differential signals.
Gain accuracy is specified over the V
SENSE
range to
keep the output voltage 250mV away from the rails to
achieve full accuracy. Output of the part is rail-to-rail
and goes to within 25mV of the rails, but accuracy is not
maintained. Linear operation is not guaranteed for input
sense voltages greater than ±150mV.
Applications Information
Power Supply, Bypassing, and Layout
Good layout technique optimizes performance by
decreasing the amount of stray capacitance at the
high-side, current-sense amplifier gain-setting pins, FB
to REF and FB to GND. Capacitive decoupling between
V
DD
to GND of 0.1µF is recommended. Since the
MAX9922/MAX9923 feature ultra-low input offset volt-
age, board leakage and thermocouple effects can easi-
ly introduce errors in the input offset voltage readings
when used with high-impedance signal sources.
Minimize board leakage current and thermocouple
effects by thoroughly cleaning the board and placing
the matching components very close to each other and
with appropriate orientation. For noisy digital environ-
ments, the use of a multilayer printed circuit board
(PCB) with separate ground and power-supply planes
is recommended. Keep digital signals far away from
the sensitive analog inputs. Unshielded long traces at
the input and feedback terminals of the amplifier can
degrade performance due to noise pick-up.
V
OUT
- V
REF
A
V
= 100
5V
2.5V
-2.5V
-25mV 0 25mV
DISCHARGE
CURRENT
CHARGE
CURRENT
V
OUT
= R
SENSE
x
(
1 +
R2
)
x I
SENSE
+ V
REF
R1
Figure 1. Bidirectional Current-Sense Transfer Function