ISL29501 Datasheet, PDF (16/23 Pages) Intersil Corporation – Auto gain control mechanism

ISL29501
System Level Calibration
The goal of calibration on the ISL29501 is to be able to calibrate
the sensor performance for different external components that
are paired with the sensor and ensure stable operation across
supply range and temperature.
There is no nonvolatile memory on-chip and the user will have to
use the I²C to program the register values during initialization.
Crosstalk Calibration
Crosstalk is defined as signal that reaches the ISL29501 chip
directly without bouncing of the target. This can be electrical or
optical. At close range a large return signal values crosstalk has
a minor impact on distance measurements. At the far end of the
distance range, the crosstalk might exceed the signal adding
error to measurements. The ISL29501 has the ability to do a real
time subtraction of crosstalk from the returned signal resulting in
a more accurate measurement. If the crosstalk remains
constant, this subtraction is very effective. This is normally a
one-time calibration performed at the factory in controlled
conditions.
For this calibration, the user makes a distance measurement
with the return signal blocked from reaching the photodiode.
Since the chip sees none of the emitted signal anything received
is crosstalk. With little to no signal, Gaussian noise will dominate
these measurements. To eliminate this noise the crosstalk
measurement needs to be averaged. The averaged value is then
written into the chip where it will be subtracted real time from all
succeeding distance measurements.
A detailed description of registers is provided in the “Register
Map” on page 17, Registers 0x24 to 0x2b hold the crosstalk
calibration values. If these registers remain at default values
(0x00) no correction will occur.
Distance Offset Calibration
Variation in delay of emitter types, photodiodes and circuit board
design will change the signal path delay. To compensate for this,
a reference point at a known distance needs to be established.
This reference is calculated during distance calibration. The
process involves making a distance measurement at a known
distance, subtracting that distance from a raw measurement and
writing the difference into the distance calibration Registers
0x2F/0x30.
Once these calibration registers are written all succeeding
distance will have this value subtracted real time from the
measured value. To insure the correct value is subtracted an
average of several measurements needs to be done. Depending
on the emitter and photodiode choice this calibration should be
required once per family (emitter/photodiode/board) type.
Optical System Design
Considerations
A system designed with the ISL29501 requires that emitter and
detector are optically isolated for better performance. There
needs to be a physical barrier or isolation between the emitter
and detector to minimize direct optical signal coupling between
emitter and detector.
GLASS
EMITTER
BAFFLE
PCB
DETECTOR
FIGURE 15. SIMPLIFIED OPTICAL SYSTEM
If a glass or other material is placed above the emitter detector,
light from the LED can reflect off the glass and enter the sensor.
This can limit the range of the proximity measurement and
manifests as faint objects in measurements.
Careful attention must be paid to some of the following design
parameters:
• Spacing between emitter and detector
• Optical isolation between emitter and detector
• Distance of the PCB from glass or from optical co-package
The ISL29501 architecture rejects most ambient optical
interference signals that are lower or higher than the modulation
frequency. A review of the ambient sources in the system will
help you understand the amount of ambient light and the
impacts on the precision measurements.
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FN8681.3
June 29, 2016