Initial accuracy of VL6180X (i.e. after reflow)

[SytseReitsma]

Hi,

We have a board featuring 5 VL6180X sensors and I noticed some oddities regarding the initial offset and scale in the latest production run:

• Some sensors are unable to measure any distance below 15mm
• The ranging linearity between different sensors is +/- 10mm (taken as the delta between a 20mm and a 68mm measurement to a matte aluminium plate).
• Initial offset is varying +/- 10mm

The initial offset appears to be roughly in spec with the datasheet (Table 18. Ranging specification), although we have quite a few sensors at the maximum offset (and in the datasheet this is after 3 reflow cycles, we only have 1).

The linearity is far worse than what I see in "Figure 6. Typical ranging performance".

The largest issue for us is the inability to measure below 15mm. Some sensors simply register 0mm (4 boards out of a batch of 30 had this issue).

I was wondering if this is the expected behavior of the sensors, or if we're doing something wrong in our design/production?

Regards,

Sytse

FYI, our design: 

[John E KVAM · Accepted solution]

There is nothing wrong with your schematic. The sensor does not depend on calibrated voltages or light intensity. These ToF chips are measuring when the photons come back and as long there are enough returning, out pops an answer. 

One thing to check is the status return. If you don't get enough signal the status will tell you. And there are other status results as well. 

The sensor will not return a negative number. Our customers hated that. But if the sensor thinks the answer should be 1 cm and you have a -11mm offset, it will return 0 instead of negative one. 

I'm guessing your offset might be too low. Or your crosstalk is wrong. 

You didn't say what kind of coverglass you are using, but perhaps a closer examination of your procedure is in order. 

Have a look at: 

https://community.st.com/t5/mems-and-sensors/time-of-flight-cover-glass/ta-p/49259

See if that gives you some insight. 

- john

[SytseReitsma]

Thanks,

I will investigate further. We do not use a cover glass.

Sytse