AAIC Homework 3
Due Fri. Apr. 14, 2006 by Tai-Cheng Lee
Please use the course SPICE device model for your simulation.
1. In this problem, we study the effect of a nonideal voltage reference upon the settling behavior of a 6-bit segmented DAC. The circuit is shown in Fig. 4.23 of the textbook. Assume each switch has a W/L = 3 µ m/.18 µ m, each capacitor is 0.5 pF, and the supply voltage is 1.8 V. Instead of the reset switch, use the .ic command to set the initial condition to zero in HSPICE.
(a) If the reference voltage is ideal and equal to 0.8 V, what is the simulated settling time to 0.5 LSB in a 10-bit system?
(b) Suppose the reference is generated using the following circuit. Obtain the minimun M4 size
V M M 1 2 REF VDD M3 M5 M 4 10/0.18 10/0.18 200 uA 500 uA W/L=30/0.18 0.8 V
(Hint: M4has to be in saturation region) by simulation. What is the small-signal output impedance of the circuit? What would this suggest about the worst-case settling time of the above DAC if it uses this reference?
(c) Simulate the DAC with the above reference generator and measure the settling time. Explain the discrepancy between this result and the estimate in (b).
(d) Suppose VREF is provided externally from an ideal source but with 3 nH of series inductance. What is the settling time in this case? Add a resistor in series with the inductor and see how much you can improve the settling?
(a) If M1–M4 have the same sizes, derive the equivalent gds for M1/M2 pair and M3/M4 pair, M1 M3 M M V V Vinp Vinn Vout CLK 2 4 refn refp VDD
respectively. Assume Vin has common-mode voltage at Vcm and Vrcm and M1–M4 are in linear region when CLK is low. Identify the characteristics of the comparator and determine if we need to have the same Vcm and Vrcm to allow the circuit work properly.
(b) Now, let’s change the ratio of M1,2 to M3,4 from 1 to 4. Please identify the characteristics of the comparator (Vin vs. Vout).
(c) Now, use HSPICE to perform the follow simulation to see the resolution of the comparators. In this part, you can set Vcm= Vrcm = 0.9V and Vref p− Vref n= 0.3V . All dimensions of transistors are 5/0.18 except M1 and M2 are 20/0.18.
(d) If we change Vcm to 0.6 V, repeat (c) and find out the resolution of the comparators. Explain it if you see different results.
