[time-nuts] V standards

[WarrenS]

Dick

Here is one way to add 2 LS  bits (or more) to any Dac no matter how it is made.
This assumes that the Dac that you are adding these bits to is 0 to 5 volts Fullscale 
and accurate enough to justify adding LS bits.

To add 2 LS bits to a 14 bit Dac.
An easy ways to do it is NOT to use PWM.
That way it needs no filtering and no PWM 
and few software resources, maybe none. 
The added 15th Bit wants to add  1 / 32,768 of 5 volts 
The added bit #16 wants to add 1 / 65,536 of 5 volts 

As a universal solution, Add a small resistor at the output of the 14 bit Dac 
of Value R (say 10  to 100 ohms).
>From one of your spare Digital  I/O micro processor pins that will become 
the 15th bit, connect a resistor value of 32,768 *R (320K) from I/O pin to the 10 Ohm 
>From your other spare Digital I/O pin used as the 16th bit,  add a 65,500*R  I/O to output. 
Standard 1% resistors are way more than accurate enough.

If you need more than three Bits, It probable pays to use PWM. 
To add a PWM use Two equal value resistor in series, 
with a  total value of 16,384*R connected to a   
PWM signal of any freq and add a filter cap between their center and ground 
to filter the small amount of noise that the PWM will be adding.

Note:
For most circuits you do not need to actually add the output resistor, 
there is always some places to add a little offset to.
All you need to do then, is find a place that you can change the offset 
value of the DAC output, and add a small filtered current into it thru a resistor / filter divider.
The total zero range you need to add will depend on the number of Dac bits you have.

As an example to expand a 5 volt 14 bit Dac, to 16 (or to 24) bits, connect 
the additional Dac thru a RCR filter to a point that will change the 
Main Dac's output to offset by up to 0.3051 mV (minus the value of one of the added bits)

One way to turn a Really good 18 bit Dac into 24 + bits, is scale a 6 bit Dac 
thru a resistor to some place on the 18 bit dac that will add 0 to 18.78 uV 
+ - < 1/2% of offset onto the 18 bit Dac's output.

Have Fun
WarrenS

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Poul, how do I PWM a DAC that is itself a PWM device? This question  
is prompted by the DAC I'm using in Bert Zauhar's FLL GPSDO, which is  
a dithered 10-bit DAC in a PIC chip that results in 14-bit  
resolution. I'd like to have a simple way to get 2 to 4 extra bits of  
resolution out of this. Can you recommend some design resources to do  
this?

Dick Moore