TDC7200_15 Datasheet, PDF (39/50 Pages) Texas Instruments – TDC7200 Time-to-Digital Converter for Time-of-Flight Applications in LIDAR,Magnetostrictive and Flow Meters

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1800
1600 ^{8 = 82 ps}
1400
1200
1000
800
600
400
200
0
-0.26 -0.20 -0.14 -0D.0e8lta-t0i.m02e-o0f-.0fl4ight0(.1n0s) ^{0.16 0.22 0.28}
Figure 47. Raw Calibrated Data Histogram
TDC7200
SNAS647C – FEBRUARY 2015 – REVISED AUGUST 2015
1600
1400
8 = 31 ps
1200
1000
800
600
400
200
0
-0.09 -0.07 -0.05 -0D.0e3lta-t0i.m01e-o0f-.0fl1ight0(.0n3s) ^{0.05 0.07 0.09}
Figure 48. 10x Running Average Data Histogram
9.3 Post Filtering Recommendations
For application such as flow meters where conversion results are accumulated over a long period of time, post
filtering is not required. However, for applications where a specific action is taken based on individual conversion
results, post filtering is recommended. One advantage of post filtering is to remove the conversion results that
are outside of the normal distribution.
One such post filtering method commonly applied by an MCU is the Median Filter Method. The median of a finite
number of conversion results can be found by arranging all the conversions from the lowest value to the highest
value, and picking the middle one. For example, a conversion result of {50, 51, 49, 40, 51} can be rearranged
from lowest to highest {40, 49, 50, 51, 51}, and the median value after applying the Median Filter Method is 50.
9.4 CLOCK Recommendations
A stable, known reference clock is crucial to the ability to measure time, regardless of the time measuring device.
Two parameters of a clock source primarily affect the ability to measure time: accuracy and jitter. The following
subsections will discuss recommendations for the CLOCK in order to increase accuracy and reduce jitter.
9.4.1 CLOCK Accuracy
CLOCK sources are typically specified with an accuracy value as the clock period is not exactly equal to the
nominal value specified. For example, an 8 MHz clock reference may have a 20 ppm accuracy. The true value of
the clock period therefore has an error of ±20ppm, and the real frequency is in the range 7.99984 MHz to
8.00016 MHz [8 MHz ± (8 MHz) x (20/10⁶)].
If the clock accuracy is at this boundary, but the reference time used to calculate the time of flight relates to the
nominal 8 MHz clock period, then the time measured will be affected by this error. For example, if the time period
measured is 50 µs, and the 8MHz reference clock has +50ppm of error in frequency, but the time measured
refers to the 125 ns period (1/8 MHz), then the 50 µs time period will have an error of 50µs x 50/1000000 = 2.5
ns.
In summary, a clock inaccuracy translates proportionally to a time measurement error.
9.4.2 CLOCK Jitter
Clock jitter introduces uncertainty into a time measurement, rather than inaccuracy. As shown in Figure 49, the
jitter accumulates on each clock cycle so the uncertainty associated to a time measurement is a function of the
clock jitter and the number of clock cycles measured.
Clock_Jitter_Uncertainty = (√n) x (θ_JITTER), where n is the number of clock cycles counted, and θ_JITTER is the
cycle-to-cycle jitter of the clock.
For example, if the time measured is 50 µs using an 8 MHz reference clock, n = 50 µs/(1/8 MHz) = 400 clock
cycles. If the RMS cycle-to-cycle jitter, θ_JITTER = 10 ps, then the RMS uncertainty introduced in a single
measurement is in the order of (√n) x (θ_JITTER) = 200 ps.
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