[volt-nuts] 32-Bit PWM divider

Andreas Jahn Andreas_-_Jahn at t-online.de
Thu Oct 7 21:54:21 UTC 2010


Hello randy:

> I'm wondering what your reason was for taking the measurements this way.

This has something to do with how the two ranges are mixed. (not added).
I simply want to get exact voltage divider ratios (2:1, 4:1 and so on) 
without knowing the exact gain for the two stages.
The resistors are cheap 0.1% 15ppm/K RC55Y-resistors.
I have 51R + 1Meg for PWM H + L as resistors .
Giving nearly 2 Bits of overlap related to PWMH.
The formula for the output voltage is as follows:

VREF * (PWMH * (1Meg + RDS,on) + PWML * 51R) / (1Meg + RDS,on + 51R) / 65536

each PWMH step has around 76uV whereas the  total PWML range is about 255uV.

so when putting PWMH = 0x8000 and having PWML = 0x0000 with VREF = 5000mV
I  get VREF/2 - 127,5 uV. = 2499.8725 mV.
With PWMH  = PWML = 0x8000 the output is exact VREF/2. (all calculated 
without INL error)

I think the best method for me to generate exact divider ratios will be the 
following.
- determine INL-error for PWMH = PWML.
- generate some correction formula for this INL term
- Output the nearest PWMH - value assuming PWMH = PWML
- correct the last microvolts (+/- 38uV) on PWML-value

Since with my measurement equipment I will neiter be able
to determine the exact (temperature dependent?) ratio of
PWMH to PWML nor to calculate any (sub uV) INL values for the
PWML-range I want to do only small corrections with the
PWML-value assuming that the PWML-range has no own INL error.

With best regards

Andreas




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