1. Switched Capacitor Blocks
Alex Doboli, Ph.D.
Department of Electrical and Computer Engineering
State University of New York at Stony Brook
©Alex Doboli 2006

2. Overview of the Chapter
Introduction to SC circuits
Programmable SC blocks in PSoC
SC principle: controlled movement of charge
Electrical nonidealities: circuit nonidealities, non-zero switch resistance, channel charge injection, clock feedthrough
Basic SC blocks: gain amplifier, programmable gain amplifier, comparator, integrator, differentiator
PSoC’s programmable SC blocks:
Type C and Type D SC blocks
Programming (registers)
©Alex Doboli 2006

3. Introduction to SC Techniques
Integrated capacitors are easier to fabricate than resistors
Average resistance approximated through charge movement
I = V / R
Q = C V
Iaverage = Q fs = C V fs => Req = 1 / C fs
©Alex Doboli 2006

4. Introduction to SC Techniques
Constraints:
Switches Ф1 and Ф2 can never be closed at the same time
Switch Ф1 must have time to open before switch Ф2 closes
Switch Ф2 must have time to open before switch Ф1 closes
Frequency fs must allow enough time for the circuits to fully charge and discharge
©Alex Doboli 2006

5. Non-idealities in SC Circuits
Non-zero on-resistance of MOSFETs:
d Vc(t) / d t = ID(t) / C
Linear: d Vc(t) / d t = m Cox W [(VDD – Vc(t) - Vth)(Vin – Vc(t)) – (Vin – Vc(t))2 / 2] / 2 LC
Vc(t) = (2 K exp (A Vin t) – A exp (A Vin t) + exp (kt + K[1])) Vin /
(A exp (A Vin t) + exp (K t + K[1])
A = m Cox W / 2 L C
K = A (VDD – Vth)
Saturation:d Vc(t) / d t = m Cox W [(VDD – Vc(t) - Vth)(Vin – Vc(t)) – (Vin – Vc(t))2 / 2] / 2 LC
Vc(t) = [(A t – C[1]) (VDD - Vth) - 1] / (A t – C[1])
Vc(0) = 0 => C[1] = - 1 / (VDD - Vth)
Vc(t) = VDD – Vth – 1 / (a t – C[1])
©Alex Doboli 2006

6. Non-idealities in SC Circuits
Channel charge injection:
Qchannel = W L Cox (VDD – Vin - Vth)
D Vc = W L Cox (VDD – Vin - Vth) / C
Trade-offs:
Accuracy vs. speed (small W helps accuracy but decreases speed)
©Alex Doboli 2006

7. Non-idealities in SC Circuits
Clock feedthrough:
Capacitive coupling through Cgd
D Vout = - Cgd,2 D VΦ2
Trade-offs:
Accuracy vs. speed (small W lowers coupling but lowers speed too)
©Alex Doboli 2006

8. SC Fixed Gain Amplifier
Characteristics:
acquisition phase
transfer phase
©Alex Doboli 2006

9. Acquisition & Transfer Phase
Q = Vin CA
Vout = - D Q / CF
Gain = - CA / CF
©Alex Doboli 2006

10. Autozero Adjustment QAi = Voffset CA QAi + QFi = QAf + QFf
QFi = Voffset CF
QAf = (Vin – Voffset) CA
QFf = (Voffset – Voutf) CF
QAi + QFi = QAf + QFf
Voutf = Voffset – [(CA + CF) Voffset – CA(Voffset - Vin)] / CF
Voutf = - CA / CF Vin
©Alex Doboli 2006

11. SC Selectable Gain Polarity Amplifier
©Alex Doboli 2006

12. SC Comparator
©Alex Doboli 2006

13. SC Integrator
©Alex Doboli 2006

14. SC Differentiator
©Alex Doboli 2006

15. Improved Reference Selection
©Alex Doboli 2006

16. Two Bit ADC
©Alex Doboli 2006

17. Analog to Digital Conversion
©Alex Doboli 2006