Specifications

Sensors
Current, light, and proximity sensing
76 Maxim Industrial Solutions
a low-input common-mode voltage,
and the output voltage is ground
referenced. However, the low-side
sensing resistor adds undesirable
extraneous resistance in the ground
path. With the high-side principle,
the sense resistor connects in series
with the positive power-supply
voltage. Here the load is grounded,
but the high-side resistor must cope
with relatively large common-
mode signals.
High-side current-sense amplifiers
from Maxim employ a current-
sensing resistor placed between
the positive terminal of the power
supply and the supply input of the
monitored circuit. This arrangement
avoids extraneous resistance in the
ground plane, greatly simplifies the
layout, and generally improves the
overall circuit performance. The
variety of unidirectional and bidirec-
tional current-sense ICs from Maxim
includes devices with and without
internal sense resistors.
Light sensing with trans-
impedance amplifiers (TIAs)
The second most popular current-
measurement technique uses an op
amp with very low input-bias current
like a TIA, which converts the current
input into a voltage output. This
principle works for much smaller
currents with large variations like
those generated by photodiodes in
light-sensing applications.
A simple photodiode is a very
accurate transducer for sensing
light. Light sensing is used in
many different applications from
power management based on
sunlight, to sophisticated industrial
process-control applications. Since
illuminance in a given situation can
vary over a broad range (e.g., from
20klx to 100klx), a wide dynamic
range can be a key requirement for a
light sensor. An integrated solution
like the MAX9635*, which integrates
a photodiode, amplifier, and analog-
to-digital converter (ADC), provides a
dynamic range of 0.03lx to 130,000lx.
Proximity sensing with a
photodiode
While proximity sensing can be
done in many ways, using a photo-
diode provides higher accuracy and
conserves more power than other
methods. When the light hits the
photodiode, a current is generated
proportional to the strength of
the light intensity. A buffer stage
with low-input noise and high
bandwidth transfers this current to
the rest of the system. An amplifier
with low input-current noise, such
as MAX9945, provides accurate
measurements.
www.maxim-ic.com/detect
*Future product—contact factory for availability.