[volt-nuts] 32-Bit PWM divider

[Andreas Jahn]

> 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?
>
What I am actually doing is the following:
I put the PWMH = 0x8000 and PWML = 0x8000. As output I expect exactly 50% of 
the reference voltage.
(I know the formula in the article would expect 50% at PWMH = 0x8000 and 
PWML = 0x0000
  but the formula in the article is wrong as you can easyly check at 0xFFFF 
for both outputs).
I measure the reference voltage (e.g. 4999.056mV) and the output voltage 
(e.g. 2499.463 mV) with my LTC2400 ADC (integrating all measurements over 1 
minute to get the noise below 1 .. 2 uV)
I would expect  2499.528 mV so the error is -65uV.
I will do this for all steps PWMH = PWML =  0x0000, 0x1000, 0x2000, .... 
0xF000, 0xFFFF (or even finer resolution).
I will get e.g. -2 uV at 0x0000 and -8 uV at 0xFFFF and -47 uV at 0x4000.
The largest deviation from the expected value gives the maximum INL-error.


>>
> 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.
>
Perhaps I wrote not clearly enough:
The LTC1151 seems to have some protection diodes
between the positive and the negative inputs of the amplifier.
At least I can measure around 1,1 to 1,2V in each direction
in diode testing of my multimeter between the two inputs.
Which would give two diodes in series in each direction.
Since there are no series resistors on most of the inputs
some unwanted current paths exist in the cirquit on large transients.

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

I have tried this (the MAX4053 is not really cheap).
my results: (all before tweaking linearity).

HCF4053 (=CD4053)   +100uV to -200 uV linearity error.
74HCT4053                  +0 to  -600 uV linearity error!!!! (I did not 
expect this)
MAX4053A                   +0 to  -65 uV linearity error

I did not check the root cause of the error and why the HCT has more error 
than the HCF.
(Leakage current, charge transfer, RDS,on span or switching times + break 
before make tolerance).
The MAX4053A is specified much better in all parameters. And in this 
application
he has the best linearity of the 3 tested multiplexer samples.

With best regards

Andreas 
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