R2R current DAC design

[diarmuid]

Hello,

I need to design a 7bit R2R current DAC.

I am looking at the current mode R2R DAC architecture as a basis. However, I do not
want to convert the output currents to a voltage. I simply want to mirror them out.
This means I wont use an opamp at the output. I am finding this leaves me with the
situation where the reference and output nets at the base of my DAC are not always
at the same voltage, thus messing up my R-2R ratios.

Does anyone have any idea of how to overcome this, good reading material, etc.?

At the end of the day all I want is a DAC whereby my binary weighted currents are mirrored
to elsewhere in the circuit. I want also to avoid the more conventional binary weighted unit
FET approach.

Thanks,

Diarmuid

 

[Dominik Przyborowski]

Use a R2R ladder as NMOS source degeneration - it's achieving constant voltage drop on the ladder and higher output impedance.

//Edit: Now I'm notice that You don't want this solution. Look for this paper

 

[erikl]

... this leaves me with the situation where the reference and output nets at the base of my DAC
are not always at the same voltage, thus messing up my R-2R ratios.

Does anyone have any idea of how to overcome this ...

You can enforce them to have the same voltage:

 

[diarmuid]

Thanks guys.

Ill use the opamp approach to force both nets to the same voltage.

Cheers,

Diarmuid