1. 6.376 Final Project: Low-Power CMOS Measurement System for Chemical Sensors
Stuart Laval
December 8, 2003

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

3. 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)

4. 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.

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

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

7. Settling time
Settling Time ~ 8 ms

8. Sample Output Characteristics

9. 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

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

11. Questions