PmodALS SPI minimum clock rate

[InPhaseDesign]

Hi, The reference manual for the PmodALS states, "The PmodALS requires the frequency of the SCLK to be between 1 MHz and 4 MHz."

However, the TI ADC datasheet doesn't indicate any such restriction.  It indicates the recommended SPI SCLK rate is from 25kHz to 20MHz.  The PmodALS schematic appears to have direct wiring from the SCLK and SDATA pins to the PMOD connector, as well.

What is the reason the PmodALS reference manual indicates a narrower range, especially a minimum of 1MHz? 

(A limited upper limit would be understandable for rise/fall time issues from trace capacitance, etc.  Also, since the SAR ADC operates on SCLK, internal leakages could lead to more error at lower rates, but the datasheet doesn't indicate any issue with slower rates in the recommended range.)

Thanks, Scott

[BKallaher]

Scott,

On page 6 of the ADC datasheet has the clock frequency between 1 and 4 MHz with a note that this is the range that electrical performance is ensured and a wider range is possible. Our ratings use this as a guide to guarantee that the device will work as expected. It should still work at the lower and higher frequencies. Let us know if you are able to get it working at these speeds.

Regards,

BKallaher

[InPhaseDesign]

Thanks, BKallaher.  Yes, I'll let you know.  Obviously it pays to read the fine print. I was going by table 7.3 on page 4 of the TI datasheet 'recommended operating conditions.'  At the top of the Electrical Characteristics Table 7.5 on pages 5 and 6, the note seems to say 1MHz to 4MHz for the "typical" values, which would imply the MAX/MIN spec limits should still be met over all the recommended operating conditions.  However, note (3) (at the bottom) for the SCLK frequency range of 1MHz to 4MHz in this table says, "(3) This is the frequency range over which the electrical performance is ensured. The device is functional over a wider range which is specified under Operating Ratings."  So the part should be functional, but they give no indication of degradation between Table 7.5 and the full range of recommended conditions in 7.3.

Thanks, Scott

[InPhaseDesign]

The PmodALS (and TI ADC081S021) work fine at 160kHz, at room temperature and with the Arty board USB powered.  The sensor is very linear and stable.