Ultra-Precision, High-Side

Current-Sense Amplifiers

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11

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

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.

swing toward the positive supply, while negative

allows the output voltage to measure both charge and

dynamic range.

In bidirectional operation, the external voltage applied

back network between OUT, FB, and REF. This current

is simply the input sense voltage divided by the resis-

tance between FB and REF (1k

Ω 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.

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

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.

5V

2.5V

-2.5V

-25mV

0

25mV

DISCHARGE

CURRENT

CHARGE

CURRENT

R1

Figure 1. Bidirectional Current-Sense Transfer Function