Texas Instruments | Extending the Range of OPT3101 Systems | Application notes | Texas Instruments Extending the Range of OPT3101 Systems Application notes

Texas Instruments Extending the Range of OPT3101 Systems Application notes
Extending the Range of OPT3101 Systems
OPT3101 is a Time-of-flight (ToF) based optical
proximity sensor and range sensor AFE. This versatile
AFE is compatible with a wide variety of emitters and
photo diodes — some of them with integrated lens and
some without integrated lens. The range and
performance of the system is determined by the choice
of these components and the optical assembly.
The OPT3101 Evaluation Module (EVM) is an
example of such implementation where through hole
components with integrated lenses have been used.
The system level performance for the EVM can be
found as part of the this application note. For certain
applications, users may want to extend the range or to
improve the performance beyond what is offered by
the OPT3101 EVM. This document shows a few
methods that could help achieve the same. It is
important to understand that OPT3101 is an active
sensing system where an emitter directs modulated
light to the target. A portion of the reflected light is
captured by the photodiode. The received signal is
typically a very small fraction of the emitted power, this
is mainly due to the physical properties of the target
(see Figure 1).
Use larger aperture receiver optics.
With this tech note, we assume that you are familiar
with the OPT3101 device and have read this user's
guide prior to reading this tech note.
OPT3101 has an internal driver capable of driving up
to a maximum of about 170mA. This maximum limit
value is critical in identifying which components emit
the highest optical power for the given current. The
efficiency of the emitters are typically between the
range of 30% to 50%, based on the type of emitter.
LEDs are lower in efficiency, but are inexpensive and
are less stringent with eye safety specification
requirements. LEDs are extremely hard to collimate
lower than ±3º. Even under such collimated cases,
there is a stray light emitted beyond the field of view.
This results in wasting optical power that surpasses
the region of interest. On the other hand, higher
efficiency emitters like VCSELs and lasers help
improve performance by not only by being more
efficient, but also by being capable of collimating to a
very small degree (up to a few milli-radians). Unlike
LEDs, VCSELS and lasers are more stringent with eye
safety classification and certification.
Stack two or more of the emitters with together to
improve the performance (see Figure 2).
Figure 1.
Figure 2.
Most targets in real life are lambartian, reflecting the
incident power in all directions with only a small
fraction captured by the receiver. Hence, the captured
power diminishes as an inverse square of the distance
to the target. The noise and accuracy of the detected
distance is a strong function of the received signal
which limits the detection range based on the desired
accuracy and the noise.
NOTE: It is very important to appreciate
that better range and better noise
performance can be achieved by
only collecting more of the reflected
signal from the target.
The following lists a few ways this can be achieved:
• Use a high efficiency emitter. Laser or VCSEL are
a few examples.
• Stack multiple emitters.
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To ensure the voltage levels on TX0/TX1/TX2 pins are
appropriate, the high side of the voltage swing should
not exceed the reliability limit. The lower side of the
voltage swing should not drop below 0.7 V which then
limits the 170mA current and causes non-linearity
problems.With these specified constraints, the stacking
options are limited to only a few topologies. For
example, stacking N emitters improves the noise
performance by a factor of N and also improves the
range for a given noise by a factor of √N.
The following lists how challenging stacking can be
along with how stacking can be accounted for:
• The optical alignment of N emitters should ensure
the emitters’ combined field of view is within the
receiver optics’ field of view
• Layout placement and routing becomes more
stringent with more switching emitter terminals and
Extending the Range of OPT3101 Systems
Copyright © 2018, Texas Instruments Incorporated
Related Documentation
larger current loops. The utmost care needs to be
taken to shield the switching nets and loops.
When using custom lens on a photodiode with no
integrated optics, users are able choose lenses with
a large aperture to collect more light. Ideally, as
shown below, a larger aperture results in a larger
fraction of the target's reflected light that is collected
which improves the performance. Increasing aperture
by a factor of N improves performance by a factor of
N2 and improves the corresponding range by a factor
of N. However, in reality, an aperture increase comes
with a proportional focal length increase. This is a
fundamental limitation related to optics and refractive
indices of a commonly available material of which
lenses are made of.
Figure 3.
same direction of the source with a very narrow
dispersion. With such a reflection profile, the signal
received by the receive chain is several folds higher
when compared to a lambartian target which
significantly improves performance. Based on the
emitter field of view, the larger the size of retro
reflector, the more signal gain the retro reflector
OPT3101 has an un-aliased range of up to 15m.
The AFE measures the phase delay between the
continuous emitted and received wave, but the AFE is
incapable of determining the actual phase delay
between emitted and received signals. For example,
targets at 1 m, 16 m, 31 m and so on would result in
the same phase measurement.However, OPT3101
has a feature where the AFE measures the target at
two different frequencies which helps to narrow down
the real distance by utilizing measurements from both
the frequencies (as shown in the illustration below).
This method is called de-aliasing and extends the
range by several folds based on the frequencies that
are chosen. Refer to the OPT3101 ToF-Based LongRange Proximity and Distance Sensor AFE data sheet
for more information.
Figure 4.
As focal length increases for a given photodiode size,
the field of view (FoV) of the receiving system reduces.
As long as the receiver FoV engulfs the emitter FoV,
then there is no signal and performance loss. LED
based systems tend to have a wider emitter field of
view which limits the choice in wide aperture and focal
length lenses. The maximum limit of the aperture is
typically at the point of which the receiver field of view
is equivalent to the emitter field of view. An aperture
increase that surpasses the maximum limit yields
marginal improvement or no improvement because the
emitted and reflect light are not all captured by the
receiving chain.
In a laser or VCSEL based system, the ability to
collimate the emitter beam to a small degree leads to
the possibility of the receiver aperture being larger in
size in comparison to the LED based system. Even
this tends to hit a practical factory alignment limit
between the transmitting and receiving optics. With an
extremely narrow field of view for both the emitter and
receiver optics, even small change in component
mounting angles, the position significantly affect the
captured signal which makes the system very sensitive
to factory calibration. Using a photodiode that is larger
in size widens the field of view for larger aperture
lenses, but OPT3101 has a maximum photodiode
capacitance limit of 6 pF at 1 V bias which limits the
choice as well.
In some applications that have a defined target, the
retro reflector being used on the target would
significantly improve noise performance and range.
Unlike lambartian reflectors, the retro reflectors reflect
a significant fraction of the incident light back in the
To summarize, the three major methods, each with
their own merits and limitations, to improve the
performance and the range of OPT3101 based
systems are explained in this tech note. Based on the
application requirements, one or more of the illustrated
methods can be applied to achieve the desired
performance. Additionally, the OPT3101 System
Estimator tool is a great way to perform what-if
analyses and performance tradeoffs in advance before
choosing the components to build the hardware.
Related Documentation
Texas Instruments, OPT3101 ToF-Based
Long-Range Proximity and Distance Sensor
AFE data sheet
Texas Instruments, Introduction to Time-ofFlight Long Range Proximity and Distance
Sensor System Design user's guide
Texas Instruments, OPT3101 System
Estimator tool
Texas Instruments, Application of OPT3101
in Precise Distance Measurement and
Ranging Applications application report
Extending the Range of OPT3101 Systems
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