[volt-nuts] 32-Bit PWM divider

[Bruce Griffiths]

Andreas Jahn wrote:
>> 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.
>
Not really you should fit a straight line to the results first.
The INL is then the given by the largest deviation from this straight line.

A very clean board with low leakage from the supplies to the opamp 
inputs together with shielding from air currents are necessary for low 
noise.
Changing thermoemfs due to fluctuating temperature differences and 
circuit board leakage can be a significant source of low frequency noise.
>
>>>
>> 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.
>
Did you make these measurements with the opamp powered up or with V+, V- 
open, or even V+ and V- shorted together?

>
> With best regards
>
> Andreas