R-2R Based DAC Non-monotonicity problem

Hi guys,

I'm designing a segment DAC (3 bits thermometer + 5 bits binary coded). You can see the core attached. My problem is with the binary part. I know that this topology is prone to non-monotonicity however, it occurs even from xxx00010-xxx00011 transition. I think that something is wrong.

I'm simulating it via placing high resistors at current outputs like 1M ohms. (I've tried to basically placing Vdc sources at the output and checking the current information as well)...

There seems no problem with codes xxx00001, xxx00010, xxx00100, xxx01000, etc... But when most of the time where the case is at least two bits are "1" it behaves strangely. I dont know if the problem is exactly non-monotonicity actually. The output seems so. But maybe there is something I am missing about current mode operation.

I'm using TG based multiplexers for switching operation.

I dont think that it has something to do with the values of resistors in the string... I've tried a wide variety of resistors from 1k to 100k....

Thank you very much in advance.

Your current summing point is low impedance? it would have been helpful if you had included all the resistor values - what is that 1x output?
Frank

chuckey said:

Your current summing point is low impedance? it would have been helpful if you had included all the resistor values - what is that 1x output?
Frank

Click to expand...

No it is not. Current summing point sees three drains, which i guess comes in parallel and forms a rather high impedance node. I've a basic extra circuitry which realizes this ---> Iout=Iref+Iout_p-Iout_n-I1x.

This in turn enables to make the 00000000 equal to a current of 0 as the lower bound of the single output swing... All the resistors are equal and 10k let's say...

No it is not.

Click to expand...

Unfortunately, it's required for R2R DAC.

FvM said:

Unfortunately, it's required for R2R DAC.

Click to expand...

Hmm.. But what I dont understand is basically I'm trying to implement a certain paper's DAC with a different technology node. That's all... I Wonder how they could do it w/o this...