[volt-nuts] 32-Bit PWM divider

Bruce Griffiths bruce.griffiths at xtra.co.nz
Thu Oct 7 22:58:26 UTC 2010


Andreas

Andreas Jahn wrote:
>> 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.
>>
> As Randy already noted I measure the "end point INL".
> I find this more useful when calibrating the endpoints and
> then compensating the INL error by some kind of error curve.
> In my case the error curve will be calibrated not so frequently as
> the end points.
> The best fit INL would give in most cases only about half the
> value of the end point INL. So the datasheet values look
> better if the best fit INL is used.

Bad idea if there are significant endpoint discontinuities as may be the 
case with this DAC when using the relatively slow switching MAX4053A.
In most cases, one can just avoid the few (1-2) endpoint codes that are 
significantly affected.

Nonlinearity in the error integrator CMRR may also be significant for 
this DAC when using the LTC1151.
This can be avoided by using a variant wherein which all opamp inputs 
are close to zero volts thus eliminating the effect of common mode 
nonlinearity.
This either requires a more complex switching arrangement for the 
sampling capacitor or adding an inverter to the error correction loop.
In this variant the DAC gain depends on the ratio of 2 resistors.

You aren't by chance using carbon cermet or thick film resistors in your 
circuit?

>
>> 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.
> In my case the largest noise source is from the LTC2400 ADC which
> is my best measurement equipment. He has 0.3ppm = 1.5uV
> rms noise which will give about 10uVpp noise. Measurement condition
> in the datasheet is with 0V at the input. The LT1027 reference of the ADC
> contributes about 3uVpp. So practically I have about 8-20uVpp noise
> over the full 0..5V input range. The only way to get rid of this noise is
> to average many values.
>
These are the known sources of noise for the ADC and reference, leakage 
between the DAC integrator summing junctions and the power supplies may 
be an issue if you havent used guard rings or teflon standoffs or 
equivalent techniques.

>> Did you make these measurements with the opamp powered up or with V+, 
>> V- open, or even V+ and V- shorted together?
> Good point: the opamp was not powered up so I have to check wether
> I have measured the substrate diodes.
>
> With best regards
>
> Andreas
Bruce




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