12 bit R-2R ladder DAC using MOSFET

[smileykid2012]

Hi,
While designing and simulating a 12 bit R-2R ladder DAC using MOSFETs, during the transition from the bit combination 011111 111111 to 100000 000000, the analog output decreases..I have designed the op amp as a simple two stage op amp and the supply voltage used is ±1.65V.The switches in the ladder network have been replaced with MOSFETs.. When I simulated a 3 bit circuit, I got the exact output levels. As the number of bits increases, the gain error of the circuit increases..

Can anybody suggest some methods to solve this problem??

 

[BradtheRad]

There is an enormous ratio between the least significant bit (its resistance, that is) and the most significant bit.

You do not want the following device to load your network. Therefore when you select an output resistance for your LSB, it should be low enough that a low or high makes a difference to the next device's input.

Suppose you select a resistance of 100 k for your LSB. That should suit an input resistance of 1M in the following stage.

Each successive bit must have a resistance of half the previous bit.

Hence to accommodate 12 bits, your MSB will need to be 49 ohms. Can you persuade the #12 mosfet to go down to 49 ohms?

If not then you may have to get by with fewer bits.