This single–supply, high–side,
low–cost current sensing method detects load current between 50mA and 1A and
converters it to an output voltage from 0.25V to 5V. High–side sensing allows for
the system to identify ground shorts and does not create a ground disturbance on the
load.
Design Notes
DC common mode rejection ratio
(CMRR) performance is dependent on the matching of the gain setting resistors,
R2-R5.
Increasing the shunt resistor
increases power dissipation.
Verify that the common–mode voltage
is within the linear input operating region of the amplifier. The common mode
voltage is set by the resistor divider formed by R2, R3,
and the bus voltage. Depending on the common–mode voltage determined by the
resistor divider a rail–to–rail input (RRI) amplifier may not be required for
this application.
An op amp that does not have a
common-mode voltage range that extends to Vcc may be used in low–gain
or an attenuating configuration.
A capacitor placed in parallel with
the feedback resistor limits bandwidth, improve stability, and help reduce
noise.
Use the op amp in a linear output
operating region. Linear output swing is usually specified under the
AOL test conditions.
Design Steps
The full transfer function of the
circuit is provided below.
Calculate the maximum shunt
resistance. Set the maximum voltage across the shunt to 100mV.
Calculate the gain to set the
maximum output swing range.
Calculate the gain setting
resistors to set the gain calculated in step 3.
Calculate the common–mode voltage
of the amplifier to establish linear operation.
The upper cutoff frequency
(fH) is set by the non–inverting gain (noise gain) of the circuit
and the gain bandwidth (GBW) of the op amp.