[time-nuts] Papers on timing for lunar laser ranging

Attila Kinali attila at kinali.ch
Sun Jul 16 05:02:35 EDT 2017

On Sat, 8 Jul 2017 11:42:44 -0700
Tim Lister <listertim at gmail.com> wrote:

> Forgive the ignorance, but why is there a large disparity between ADC
> and DAC capabilities ?
> For example, Linear Technology sell a 24 bit ADC for ~$7 but an 18 bit
> DAC is $30-50...

Much simplified, it boils down to it being easier to measure voltage
differences by averaging than keeping a voltage constant.

E.g. in those >20bit ADC's you will usually find a delta-sigma ADC,
usually 3rd to 5th order with a 1.5 to 5 bit ADC/DAC inside. The ADC
and DAC can be laser trimmed to be in the order of 0.1% of their
ideal values. With a few additional tricks you can get the most of
the remaining non-linearity out. These tricks also help to remove
errors due to DC-offsets in the signal path. But the biggest
improvement comes from averaging over many "samples" to get the
white noise out. If you look at the usual sample rates at which
those ADC reach their "full" performance, it is around 
1-30 (output) samples per second.

On the other hand, on a DAC you need to keep the output voltage
stable. You can do the same delta-sigma approach as with the ADC
with much the same result, but you have one big problem:
it is not easy to build an analog low pass filter that has a corner
frequency down at 10Hz. This means, you have to work at a much higher
frequency to have a low pass filter that can be realized (let's say 1kHz
if you are building a discrete filter, higher if it's integrated).
But that means that you have several orders of magnitude more (white) noise.

Additionally, a lot of people expect to do a couple of 1000 samples
per second at least, to have a usefull DAC. But that contradicts the
need to have a narrow band low pass filter to get the noise out.

			Attila Kinali
It is upon moral qualities that a society is ultimately founded. All 
the prosperity and technological sophistication in the world is of no 
use without that foundation.
                 -- Miss Matheson, The Diamond Age, Neil Stephenson

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