I'm trying to convert 1.5V logic levels from a caliper data port to 3.3v to be able to read it with a microcontroller, using one of the sparkfun logic level converters. The product page mentions it can be used for 1.8V logic levels, so I was hoping 1.5V would work as well.
When I look at the waveform generated on the HV side, the high logic level is at the expected voltage, but the low logic level is much higher than expected, and I'm not sure why.
Interestingly, if I connect the LV side directly to the ground and 1.5V from the caliper's data port, the HV side shows 0V and 3.3V as expected
I have it hooked up like so. Note: the probe was actually connected to the CLK pin on both sides (LV2/HV2), not the data pin as shown. Although I got similar results when looking at the data signal.
And here are the traces from the LV side and the HV side.
(LV)
(HV)
I eventually got it working by using a rail-to-rail opamp instead, since I only need unidirectional conversion, but I'd like to understand why I was seeing unexpected results with this. And why connecting the LV side directly to ground produced different results than the low logic level from the clock/data pins.
My best guess is that maybe the clock/data pins from the caliper are unable to sink much current when in a low logic level state?
Best Answer
If level converter works when you directly connect LV input to GND or 1.5V and does not work from DATA/CLK pins then it means logic levels on those outputs do not have enough swing to switch the FET. Or maybe do not have enough sink capability.
One possible workaround is to reduce pull-up strength on that adapter board from 10K to 22K or more (on LV side only).
However more reliable solution would be to use different level shifter, something that does not depend on Vgs of the MOSFET and has high input impedance. Like PCA9306 for example.