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Texas Instruments Accurate Measurements with OPT3101 Application notes
Accurate Measurements with OPT3101
OPT3101 is a Time-of-flight (ToF) based optical
proximity and range sensor AFE which can be used to
build systems for wide variety of applications like long
range proximity, wide field of view proximity detection,
collision avoidance is autonomous robots just to name
a few. This versatile AFE is compatible with a wide
variety of photo-diodes and emitters, and can be
configured to fit different applications based on
performance and power requirements. Performance of
the system comprises of several aspects like data rate,
noise performance, power, size and distance error.
Based on the application one or more of the
performance parameters take priority over the others.
Distance error or absolute accuracy is one such critical
parameter for some applications. For more information
and to view the definition of this, refer to the
Introduction to Time-of-Flight Long Range Proximity
and Distance Sensor System Design user's guide
(Section 4.7, Figure 9 [Distance Error]). Like other
optical time of flight systems, OPT3101 systems
require factory calibration for compensating various
system level artifacts. This is a critical process to
achieve accurate measurements. Realistically, there
are several phenomenon that affects the accuracy,
impact of which is analyzed in this document. With this
tech note, we assume that you are familiar with the
OPT3101 device and have read the Introduction to
Time-of-Flight Long Range Proximity and Distance
Sensor System Design user's guide prior to reading
this tech note.
OPT3101 is a continuos time indirect time of flight AFE
which measures target distance by measuring the
phase delay between light pulses emitted and
received, fundamentals of which can be found in
Section 1 and Section 2 of system design guide.
OPT3101 measures 16 bit phase output in digital
codes or counts which is translated to distance in the
host processor. There are two constants involved in
this translation listed below
1. Frequency of modulation ➡ This translates the
measured phase in codes to delay measured in
nano seconds
2. Speed of light in medium ➡ This translates the
delay in nano seconds to distance traveled
Speed of light in air is considered a constant and is
barely affected by temperature and humidity, which
leaves us with frequency of modulation to consider.
OPT3101 uses internal oscillator, frequency of which
changes by around 4% over a temperature range
of 100°C. This directly translates to distance error
when uncompensated. Using OPT3101 continuos
frequency calibration mode, as explained in Section
4.1 of calibration guide, compensates for this error.
Error due to this phenomenon may be diminished to
around 1mm or lower when compensated
appropriately. With 16 bit phase output, quantization
error of around 0.2mm never limits system
performance; most often other error sources dominate
as explained below.
Residue from Electrical and Optical crosstalk is a
prominent source of error, especially at lower signal
amplitudes. Although crosstalk correction and
crosstalk temperature correction coefficients are
applied, it is possible that fractions of uncorrected
residue crosstalk remain in the system. This residue
crosstalk is what typically limits the range of the
system for a given accuracy specification. Technical
details for this phenomenon can be found in Section
8.1 of the Introduction to Time-of-Flight Long Range
Proximity and Distance Sensor System Design
document. From the formula for distance error, a
strong dependency towards signal amplitude can be
observed.
(1)
For example, assume a residue crosstalk amplitude of
10 codes; with a lower signal amplitude, such as 1000
codes, the error is around ±24 mm, while the distance
error with full signal amplitude (215) is around ±0.7 mm,
Therefore, it is crucial to keep the system crosstalk to
a minimum, thus enabling crosstalk correction to be
more accurate in turn minimizing residue crosstalk. For
more information, refer to the Introduction to Time-ofFlight Long Range Proximity and Distance Sensor
System Design
Temperature change causes delay changes in the
system, which in turn causes change in the measured
phase. This phenomenon is dominated by the emitter
delay changes with temperature. Compensation for
this is done by phase temperature correction digital
block using phase temperature coefficients
programmed to the device. Typically these coefficients
are common for a hardware design and same
coefficients are programmed to all units during factory
calibration. OPT3101 offers two methods for such
compensation using
• Internal Temperature sensor
• External Temperature sensor IC
The internal temperature sensor has a resolution of
1ºC , whereas with an external temperature sensor IC
like TMP102,a resolution of up to 0.0625ºC can be
achieved. Using internal temperature sensor provides
a quick response low resolution (1ºC) compensation vs
using external which provides a relatively slow
response high resolution (0.0625ºC) compensation.
Since emitters are connected to OPT3101 device pins,
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Related Documentation
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internal temperature sensor senses the emitter
temperature way faster as compared to external
temperature sensor IC. By placing external
temperature sensor very close to the emitter along
with good thermal coupling would improve external
temperature sensor response times. Typical
temperature coefficient for this phenomenon are
around 6 mm per ºC depending on the type of the
emitter and operating current, which means that with
the usage of internal temperature sensor the absolute
accuracy gets limited around 6mm.Using high
resolution external temperature sensor fractions of mm
of accuracy is achievable. Based on the application
requirements, one or a combination of the two
compensation schemes could be used to achieve
desired accuracy.
Ambient current emerging from the photo-diode is
handled by the ambient cancellation block of
OPT3101. Although OPT3101 rejects ambient current
from photo-diodes, the photo-diodes have a
phenomenon where the amount of ambient light
falling on the photo diode, affects the phase delay
of the modulated current. This can be observed at
system level as a phase offset error dependent of the
ambient light. OPT3101 has a digital correction block
to compensate for this phenomenon. For more
information about this, refer to the OPT3101 Distance
Sensor System Calibration user's guide. Typically the
phase offset error is non-linear with ambient light,
which can be compensated with four segment piece
wise linear (PWL) correction. The residue non-linear
fraction from the PWL correction could cause distance
error with ambient light changes. This is highly
dependent on the photo-diode model and the
coefficients determined, typically adds a few mm to the
distance error.
Square wave non-linearity is a phenomenon that
occurs because the light source modulation waveform
is not purely sinusoidal. This causes a period of error
in the distance measurement, based on the amount of
harmonic content in the modulated light. A typical error
is about ±20 mm with a periodicity of 3.75 m which is
1/4th of the unambiguous range. OPT3101 has a digital
correction block for compensation, where coefficients
can be programmed based on measurements in a lab
environment as part of system level calibration. Since
the digital correction is high resolution, with
appropriate coefficients, the error could be reduced to
1 mm or lower.
The field of view of the system affects the
accuracy profoundly, this is explained in section 7.1
of system design document. Wider field of view
systems inherently have higher errors due to the
following factors
• The measurement compromises of light rays from
the actual target of interest and the background,
which causes error unless the target occupies the
wide field of view system entirely.
2
•
Even if target covers the entire field of view, since
a large portion of the target contributes to the
measurement, the apparent distance measured by
various rays at different angles sum up to cause
higher error.
• Spacing between the emitter and the receiver
creates triangulation distance errors. These are
more sensitive within the field of view systems.
• Multiple reflectivity points with in target would
cause light rays to disproportionately add up
causing distance errors.
Some of these physical effects are fundamental to any
optical system which cannot be corrected, however it
is possible to measure and compensate some of these
effects as part of additional calibration on the host
application processor.
To summarize, since OPT3101 systems can be
tailored for different configurations, the accuracy
depends on several factors. The most important factor
is the system level calibration's thoroughness. A more
narrow field of view system can achieve absolute
accuracies of a few millimeters when the signal levels
are good and few centimeters when the signal levels
are low. To get the accuracy measurement to be less
than 1 millimeter, additional calibration and
compensation of the host application processor may
be required. At close distances, as a result of physical
properties of optical systems, especially where the
separation of the emitter and receiver are comparable
to the distances measured, the errors may be a few
mm due to triangulation artifacts. In such cases,
additional triangulation compensations could be
performed on the host application processor to
improve accuracy. In wider field of view systems,
definition of accuracy fails to make sense since the
target area measured enlarges rapidly with increase in
distance bringing more objects in to the field of view.
In such cases it makes sense to include the
application algorithm in to account and look at the
overall reliability of the system functionality rather tan
the OPT3101 system measurement accuracy.
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Related Documentation
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•
•
•
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
Accurate Measurements with OPT3101
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