EE 435. Lecture 38. DAC Design Current Steering DACs Charge Redistribution DACs ADC Design

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1 EE 435 Lecture 38 DAC Design Current Steering DACs Charge edistribution DACs ADC Design

2 eview from last lecture Current Steering DACs X N Binary to Thermometer ndecoder (all ON) S S N- S N V EF F nherently nsensitive to Nonlinearities in Switches and esistors Smaller ON resistance and less phase-shift from clock edges Termed bottom plate switching Thermometer coded

3 eview from last lecture Current Steering DACs T-gate Binary to Thermometer ndecoder (all ON) S S S N N V EF F CELL k β = = CELL CELL + CELL+kF F k Transistor mplementation of Switches f VOUTFS VEF CELL = NF 0.5 < β

4 eview from last lecture Current Steering DACs V EF X N n S S S 3 3 n- S n n F Binary-Weighted esistor Arrays Need for decoder eliminated! DNL may be a major problem NL performance about same as thermometer coded if same unit resistors used Sizing and layout of switches is critical Observe thermometer coding and binary weighted both offer some major advantages and some major limitations

5 eview from last lecture Current Steering DACs V EF n X N S S 3 S 3 4 S 4 5 S 5 6 S 6 7 N S 7 F Binary-Weighted esistor Arrays Actual layout of resistors is very important

6 eview from last lecture Another - DAC b 4 b 4 b 3 b3 b b b b V EF

7 Another - DAC F b 4 b 3 b b V SS

8 Another - DAC F b 4 b 3 b b b 4 b 3 b b V SS

9 Current Steering DAC n Binary to Thermometer Decoder (all ON) d k ON d d N- F OUT =k OUT Switch impedance of little concern

10 Current Steering DAC n- F X N n b n b n- b OUT =k OUT

11 Current Steering DAC X MSB n Binary to Thermometer Thermometer Coded Array X LSB n Binary Coded Array F OUT

12 Current Steering DAC n Binary to Thermometer Decoder (all ON) # k d d d N- F OUT =k OUT V DD

13 Current Steering DAC V DD n Binary to Thermometer Decoder (all ON) # k d d d N- F OUT =k OUT V DD C P

14 Current Steering DAC n Binary to Thermometer Decoder (all ON) # k d d d N- F OUT =k OUT V DD M Cascode Current Source (Mirror) V YY M Differential Amplifier (Analog) M 3 M 4

15 Current Steering DAC V DD V YY M M n Binary to Thermometer Decoder (all ON) # k d d d N- F M 3 M 4 OUT =k OUT V DD V YY M M C P M 3 M 4 C P

16 V DD Current Steering DAC V YY M M C P n Binary to Thermometer Decoder (all ON) # k d d OUT =k d N- OUT F M 3 M 4 C P D T D D D V M M V T T V d V EB V EB

17 Current Steering DAC with Supply ndependent Biasing V DD V EF X X X d0 d0 d d dn dn A N= n B f transistors on top row are all matched, X =V EF / Thermometer coded structure (requires binary to thermometer decoder) A V EF N i0 Provides Differential Output Currents d i

18 Current Steering DAC with Supply ndependent Biasing V DD V EF X X X X d0 d0 d d dn dn V A A N= n B V B X f transistors on top row are all matched, X =V EF / V V d N A A EF i i0 Provides Differential Output Voltages

19 Current Current Steering DAC with Supply ndependent Biasing V DD V EF W W W n- W X X n- X d0 d0 d d dn dn A X B f transistors on top row are binary weighted A V d n EF i ni i0 Provides Differential Output Currents

20 Matching is Critical in all DAC Considered V DD V DD b k b k V EF V EF S cn S cn S S S ck S ck Obtaining adequate matching remains one of the major challenges facing the designer!

21 Dynamic Current Source Matching k EF k C C C C Ck Ck C C C Correct charge is stored on C to make all currents equal to EF Does not require matching of transistors or capacitors equires refreshing to keep charge on C Form of self-calibration Calibrates current sources one at a time Current source unavailable for use while calibrating Can be directly used in DACs (thermometer of binary coded) Often termed Current Copier or Current eplication circuit

22 Dynamic Current Source Matching EXT k EXT k k EF CEXT CEXT C C C C Ck Ck C C C C Extra current source can be added to facilitate background calibration

23 End of Lecture 38