[SOLVED] How to improve linearity in current-output current-steering binary-weighted DAC?

[melkord]

I find that the output current is not really double of the current of the previous bit.

 

[frankrose]

Multiply it with exactly 2 .... You should share more info, this is ridiculously short, nobody can help you.

 

[dick_freebird]

Current mode DACs only work well into a "virtual
ground" of low (zero) impedance. If the VOUT is
able to move then it will bend the IOUT.

Careful design of the "innards" is needed to avoid
issues like major-carry DNL (where IOUT(MSB) <>
IOUT(MSB-1)*2.

 

[timof]

The most frequent (and most likely) reason for the linearity loss in current steering DAC, is mismatched voltage drop on the ground net. Current sources are connected to a common ground net, with finite resistance, and the flow of current causes IR voltage drop on the ground net, different for different current sources (or their combination).

Voltage drop on ground net Vs, affects Vgs = Vg-Vs, and hence a current through the transistors.

There are several root causes for this IR drop mismatch, including local and long-range.

The effect is explained well in this paper by Prof. Razavi:

The Current-Steering DAC [A Circuit for All Seasons]​

Our EDA tool, ParagonX, has a functionality to characterize, detect, find root causes, to fix such IR drop mismatch.