[volt-nuts] 32-Bit PWM divider

Andreas Jahn Andreas_-_Jahn at t-online.de
Tue Oct 5 18:50:49 UTC 2010


Hello Bruce,

> Andreas
>
> There is an large error in the resistor values that determine the gain of 
> the LS PWM DAC in the EDN article resulting in a gain mismatch between the 
> most significant 16 bit PWM DAC and the least significant PWM DAC.

How do you think that the resistor values should be?

When calculating from the diagram 15.4 Ohms * 65536 = 1009254.4 Ohm
which comes close to the serial resistance of the 1Meg + 9K2 + RDS,on of the 
MAX4053.
the other of the 1Meg is shorted always either to Gnd or to the reference.
Or am I wrong ?

http://www.edn.com/article/471981-DC_accurate_32_bit_DAC_achieves_32_bit_resolution.php

> The resultant linearity error is much larger than the effect of the 
> difference between the 5 ohms and the 7.15 ohms you used.
> This gain mismatch will result in the ~ 16 bit nonlinearity error that you 
> observed.
>
> Bruce

In my linearity measurements I always used the same values for the high and 
low PWM-Value.
So I think that the gain mismatch should cancel out in this special case.

When I have correctly understood the 5 Ohms should cancel out the RDS,on for 
the switch S1
which is higher when the voltage across the switch is near +5V and lower 
when the voltage
is near 0V. The RDS,on (high and low) and R7 are in series to the 
integration resistor R1 (88K7)
so each Ohm mismatch will result in around 6 ppm error at 50% duty cycle.
Unfortunately the RDS,on is not temperature independent.

With best regards

Andreas





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