VL6180X datasheet(23/79 Pages) STMICROELECTRONICS

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VL6180X

Functional description

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

•

enable

•

range or ALS selection

•

clear buffer

SNR

RESULT__RANGE_RETURN_SIGNAL_COUNT{0x6C}

RESULT__RANGE_RETURN_AMB_COUNT{0x74} * 6

--------------------------------------------------------------------------------

Photon detectors - Single Photon Avalanche Diodes

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).

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