拉扎维《模拟集成电路设计》第二版课件 Ch13.ppt

MOSFETS as Switches: Precision Considerations * Speed trades with precision Channel Charge Injection: For MOSFET to be on, a channel must exist at the oxide-silicon interface Assuming Vin ? Vout, total charge in the inversion layer is When switch turns off, Qch exits through the source and drain terminals (“channel charge injection”) MOSFETS as Switches: Precision Considerations * Charge injected to the left is absorbed by input source, creating no error Charge injected to the right deposited on CH, introducing error in voltage stored on capacitor For half of Qch injected onto CH, error (negative pedestal) equals MOSFETS as Switches: Precision Considerations * If all of the charge is deposited on CH, Since we assume Qch is a linear function of Vin, circuit exhibits only gain error and dc offset MOSFETS as Switches: Precision Considerations * Clock Feedthrough: MOS switch couples clock transitions through CGD or CGS Sampled output voltage has error due to this give by Cov is the overlap capacitance per unit width Error ?V is independent of input level, manifests as constant offset in the input/output characteristic MOSFETS as Switches: Precision Considerations * kT/C Noise: Resistor charging a capacitor gives a total RMS noise voltage of On resistance of switch introduces thermal noise at output which is stored on the capacitor when switch turns off RMS voltage of sampled noise is still approximately equal to Charge Injection Cancellation * Charge injected by main transistor removed by a dummy transistor M2 M2 is driven by so that after M1 turns off and M2 turns on, channel charge deposited by M1 on CH is absorbed by M2 to create a channel If W2 = 0.5W1, then charge injected by M1, ?q1 is equal to that absorbed by M2 * Charge Injection Cancellation If W2 = 0.5W1 and L2 = L1, effect of clock feedthrough is suppressed Total change in Vout is zero because * Charge Injection Cancellation Incorporate both PMOS and NMOS dev