[volt-nuts] 32-Bit PWM divider

[Bruce Griffiths]

Andreas

Andreas Jahn wrote:
> 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?
I will answer in more detail tonight (my time when I dig up my analysis 
notes on this) and include an analysis of acquisition transient currents 
in the 1uF sampling capacitor as a function of MS PWM duty cycle, etc.

>
> 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 ?
The Thevinen equivalent source  at pin 15 of the MAX4053A has a series 
resistance of  0.5M + 9K2 connected to 2.5V.
The 9K2 resistor should actually have a value of 4K6.
>
> 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.
>
How do you make linearity measurements with both the MS and LS PWM 
values fixed?
Surely you mean that you kept the LS PWM value fixed whilst varying the 
MS PWM value?

> 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
>
Your comments on forward biasing the analog switch protection diodes are 
somewhat perplexing as the analog switch negative supply (Vee) should be 
-5V.
1.5V transients are well within the resultant +5V to -5V range for which 
the analog switch protection diodes are reverse biased.

Unless of course you connected the analog switch Vee to GND.

The circuit should also work with an HC4053 substituted for the MAX4053.

Bruce