User manual
DocID026171 Rev 6 23/79
VL6180X Functional description
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2.8.3 Signal-to-noise ratio (SNR)
In high ambient conditions range accuracy can be impaired so the SNR threshold is used as
a safety limit to invalidate range measurements where the ambient/signal ratio is considered
too high.The default ambient/signal ratio limit is 10 (i.e. an SNR of 0.1) which is then
encoded in 4.4 format as follows:
SYSRANGE__MAX_AMBIENT_LEVEL_MULT{0x2C}= 10 x 16 = 160
To enable the SNR check, set bit 4 in SYSRANGE__RANGE_CHECK_ENABLES (0x02D). A
lower setting results in a more aggressive filter which will result in a lower effective range but
greater accuracy. A higher setting results in a less aggressive filter which will result in a
greater effective range but lower accuracy.
The SNR value can be calculated as follows:
Note: The SNR value is the inverse of the ambient/signal ratio limit {0x2C}.
Note: The optimum value for SNR threshold should be determined in the final application.
2.9 Manual/autoVHV calibration
SPAD
(e)
sensitivity is temperature dependent so VHV
(f)
calibration is used to regulate SPAD
sensitivity over temperature in order to minimize signal rate variation. VHV calibration is
performed either manually by the host processor or automatically by internal firmware.
Execution time is typically 200 μs so has no impact on normal operation.
A VHV calibration is run once at power-up and then automatically after every N range
measurements defined by the SYSRANGE__VHV_REPEAT_RATE{0x31} register.
AutoVHV calibration is disabled by setting this register to 0. Default is 255. If autoVHV is
disabled it is recommended to run a manual VHV calibration periodically to recalibrate for
any significant temperature variation. A manual VHV calibration is performed by setting
SYSRANGE__VHV_RECALBRATE{0x2E} to 1. This register auto-clears. This operation
should only be performed in software standby.
2.10 History buffer
The history buffer is a 8 x 16-bit memory which can be used to store the last 16 range
measurements (8-bit) or 8 ALS samples (16-bit). Use of the history buffer is controlled via
register SYSTEM__HISTORY_CTRL{0x12}. There are 3 basic functions:
• enable
• range or ALS selection
• clear buffer
SNR
RESULT__RANGE_RETURN_SIGNAL_COUNT{0x6C}
RESULT__RANGE_RETURN_AMB_COUNT{0x74} * 6
---------------------------------------------------------------------------------=
e. Photon detectors - Single Photon Avalanche Diodes
f. VHV is an adjustable SPAD bias voltage and stands for Very High Voltage (typically around 14 V). Also
sometimes referred to as CP (Charge Pump).