Why my DAC became not monotonic ?

[wany]

Why DAC is not monotonic

i am modifying a 6bit R-2R ladder DAC to 8bit DAC. why only add up two resolution , it become not monotonic? thx

 

[Vamsi Mocherla]

Re: Why DAC is not monotonic

When you increase the resolution by 2 bits, the number of sources should be quadrapled. Did you do that?

 

[wany]

Re: Why DAC is not monotonic

the number of source should be quarable. the source is digital signal.right?

 

[jiangwp]

Re: Why DAC is not monotonic

Maybe the nonmonotonic is exist in the changing for 01111111 to 10000000 or some other codes.

the nonmonotonic is appear due to the nonlinearity accumulation , other offsets , mismatch and load change.

you can improve the layout match for resitance.

or change the parameters of buffer(OPAMP) such as Gain , BandWidth, SlewRate.

All way that is improve the linearity of DAC system is valid to improve the monotonic.

 

[Vamsi Mocherla]

Re: Why DAC is not monotonic

In this case, the most probable thing could be the gain of your opamp and its settling. Is your DAC binary scaled?

 

[10kangstroms]

Re: Why DAC is not monotonic

i am modifying a 6bit R-2R ladder DAC to 8bit DAC. why only add up two resolution , it become not monotonic? thx

As the number of bits goes up, the tolerance of the resistors must go down. An 8 bit DAC requires tolerances that are 4 times as stringent as a 6 bit DAC.

 

[Vamsi Mocherla]

Re: Why DAC is not monotonic

Yes, it could also be because of the resistor tolerances. Please check your resistor model file. In case, the W of the resistors are small, then you can double them or make them 4 times the current width. Do not forget to scale the lengths in the same manner.

 

[wany]

Re: Why DAC is not monotonic

Thank you very much!

 

[wany]

Why DAC is not monotonic

Now my the gain and bandwidth is enough large. But it become giltch when code chang from 01111111 to 10000000 and some other code. how can i resolve this problem

Added after 1 minutes:

Now my the gain and bandwidth is enough large. But it become giltch when code chang from 01111111 to 10000000 and some other code. how can i resolve this problem

Added after 50 minutes:

Now my the gain and bandwidth is enough large. But it become giltch when code chang from 01111111 to 10000000 and some other code. how can i resolve this problem

 

[Vamsi Mocherla]

Re: Why DAC is not monotonic

Well, the reason for that is because your DAC is in binary mode. To reduce the glitches, you have to put the top few bits in the thermometric code. If you are designing a 6 bit DAC, you can put 3 bits in binary and 3 in thermometric.

At the device level, you should take care of your switch sizes. Because of charge feedthrough, there will be glitches due to the charge injection.

Finally, see whether you are using proper clocks. Depending on the architecture, you should see whether you have a overlapping or a non-overlapping clock

 

[wany]

Re: Why DAC is not monotonic

Are there any methods excpet for theremometric code. And i have another question: why the result of output (analog signal) is bigger and bigger than ideal ,when code from 11111111 to 0000000. thank you

 

[Vamsi Mocherla]

Re: Why DAC is not monotonic

In a real world condition, the offset and the gain error come into play which can move the ouput signal either way. Do not worry about them. The only concern should be if there is a very huge offset. Gain error can be reduced by tweaking the sizes of your caps.

 

[wany]

Re: Why DAC is not monotonic

Are there any methods excpet for theremometric code. And i have another question: why the result of output (analog signal) is bigger and bigger than ideal ,when code from 11111111 to 0000000. thank you

 

[Vamsi Mocherla]

Re: Why DAC is not monotonic

I am sorry about the earlier post. I thought you are designing charge redistribution DAC. In case of a R-2R circuit, please check the values of Rf(whether it is optimized for exact gain) and the currents through the R side.

 

[10kangstroms]

Re: Why DAC is not monotonic

Are there any methods excpet for theremometric code. And i have another question: why the result of output (analog signal) is bigger and bigger than ideal ,when code from 11111111 to 0000000. thank you

Sample and hold the output after the glitch has settled. I did this back in the '70's, before good video DACs were available. It works well.

 

[wany]

Re: Why DAC is not monotonic

the value of Rf ,what is Rf mean?

 

[Vamsi Mocherla]

Re: Why DAC is not monotonic

I am sorry...Rf is the feedback resistor on your opamp

 

[wany]

Re: Why DAC is not monotonic

thank you for your help . i still have a problem . there are three poins of my analog output can't settle. how to sovle it .