6.376 Final Project: Low-Power CMOS Measurement System for Chemical Sensors Stuart Laval December 8, 2003

Outline Project Goal Total Chip Schematic Op-Amp/Comparator Transfer Characteristics Step Response Output Characteristics Specifications Chip Layout

Project Goal Design a low-power chip for measuring the resistive change in a chemical sensor. Wheatstone Bridge Null-detector Immune to Power Supply variations Sensor Characteristics Nominal Resistance ~ 100K Ohms Maximum change ~ 1K Ohms Target resolution ~ 100 Ohms (0.1%) System Requirements Power < 20 mW Vdd = 3 V Voltage change resolution ~ 0.75 mV Gain > 4000 (72 dB)

Total Chip Schematic Auto-zero circuit nullifies input offset error Clk = 1, Switch closed, V- stored on cap Clk = 0, Switch open, V+ compared to ref LEDs indicate the number of 100 Ohm level changes.

Op-Amp / Comparator M=2 M=2 M=2 Ibias = 1nA M=2 M=2

Gain Bandwidth Gain ~ 70000 (97 dB) Comparator GB ~ 40 KHz Op-amp GB ~ 750 Hz Phase Margin ~ 60° 20 dB/dec 40 dB/dec

Settling time Settling Time ~ 8 ms

Sample Output Characteristics

Specifications Ibias = 1 nA Gain ~ 97 dB Bandwidth ~ 750 Hz Settling Time ~ 8 ms Analog Power = 10.1 nW Total Power = 125 uW Total Noise = 80 nV Ibias = 40 nA Gain ~ 97.3 dB Bandwidth ~ 27 KHz Settling Time ~ 350 ms Analog Power = 391 nW Total Power = 125 uW Total Noise = 82 nV

Chip Layout (40 nA Iref) Op-Amp in Common Centroid Total Chip Layout

Questions