Mixed-Signal Processing Unit
for Sensing Signal Processor
NCTU TWT Lab
2011/11/18 (Update)
SSP Platform Architecture
Sensing Signal Processor (SSP)
RTC
Sensor B
Sensor C
AFEuni
Aux port
Ext.
Sensing
Actuating
Module
Aux port
Sensor A
uP
Storage
RF
USB
Ext Storage
Sensor D+AFE
Sensor E+AFE
Power
Module
uP: 8051/Open RISC
RF: Bluetooth
USB:USB 1.0
Proposed MSPU Block Digram
• Universal LNA: Gain 2x / 20x
• PGA: Gain 1x, 2x, 4x, 8x, 16x
• Filter: TBD
• ADC Freq: 10kHz max
• ADC Resolution: 12 bit, 0.8V (FS)
• System power supply: 1.2V
(Revised 11/18/2011)
Buffer
志峻
Temperature
執中
Acc.
執中
Pressure
居正
Humidity
Universal
Int. sensor
LNA
Differential
Cap. sensing
昱賢
PGA
Gain
Control
Ext. Ch1
Ext. Ch2
D-S
ADC
To
MCU
居正
Universal
Ext. sensor
LNA
CMRR >100dB
Ext. Ch3
Ext. ChN
Filter
Gain
Control
Differential
Volt/Cap. sensing
Gain
Control
Gain / Filter / ADC
Control Logic
SPI
To
MCU
Programmable Internal Sensor LNA

The system shall support at least





1x accelerometer (執中)
1x pressure sensor (居正)
1x humidity sensor (0~100% RH, 2% res.-居正/物聯網規
格)
1x temperature sensor (-20⁰C ~ 60⁰C, 0.5 ⁰C res. .-志峻/
物聯網規格)
Sensing capacitance for pressure and humidity
sensors



Humidity – 6~7pf with 20% variation on humidity
Pressure - <1pf with 5~10% variation on pressure
Both humidity and pressure sensor are pure capacitive
sensor with single end interface
 Need calibration / trim to get accurate measurement
 Read-out mechanism needs further study
Programmable External Sensor
LNA

The system shall support at least





2 channel differential input for 3-lead ECG or auxiliary
differential senor input
2 channel auxiliary differential input for touch sensing
ECG voltage range 0.1~4mV
EEG voltage range 10µV~200µV
Touch/proximity capacitive sensor



CS = 5~20pF
Sensitivity @ 15fF (ST) or 1fF (ADI)
With 12-bit ADC, it’s ok for 15fF spec, but difficult to
achieve 1fF sensitivity without any improvement on circuit
design  need further study
ADC Calculation


Consider ADC spec is 0.8 V full scale
with 12-bit resolution (昱賢)
Acceelerometer (執中)


Sensor sensitivity at LNA input can be
as low as 1mV/g
Consider the application for +/-2~16g
applications



+/-16g = 32g x 1mV/g = 32mV (gain < 25)
+/-2g = 4g x 1mV/g = 4mV (gain < 200)
Choosing DC gain 20x/160x and 12-bit ADC




+/-16g: 1mV/g x 20 * (2.048LSB/mV) ~= 41 LSB/g >16
+/-2g: 1mV/g x 160 * (2.048LSB/mV) ~= 320 LSB/g >256
Good enough for mobile application
BOSCH G-sensor
(Used in mobile application)
Humidity and Pressure sensor (居正)

Humidity: 6~7pF with 1.2~1.5pF variation



Consider 0~10pF range with 12-bit resolution  2.4fF / LSB
For 1.2~1.5pF variation  500~625 LSBs ~= 9-bit resolution  OK for 物聯網
Pressure: <1pF with 0.05~0.1pF variation


Consider 0~1pF range with 12-bit resolution  0.24fF / LAB
For 0.05~0.1pF variation  200~400 LSB ~= 8-bit resolution
ADC Calculation (cont.)

ECG (富彥 / LNA 執中)

Input range – 0.1~4mV







Max gain < 200 (4mVx200=0.8V full swing ADC range VFS)
Required SNR >=32dB for a good ECG analysis
12-bit ADC ~=74dB SNR  consider 1-bit loss  68dB SNR
For 4mV input & 160x gain  640mV / 800mV = -2dB VFS
 -2dB VFS + 68dB = 66dB SNRADC > 32dB (OK!)
For 0.1mV input & 160x gain  16mV / 800mV = -34dB VFS
 -34dB VFS + 68dB = 34dB SNRADC > 32dB (marginal !!)
For small input ECG, need to increase the gain
 LNA stage = 20x / PGA gain = 16x
 total gain = 320  SNR = 40dB SNRADC (good enough!)
EEG



Consider the same setting as ECG but input range 10uV~200uV
Required SNR >=20dB for good EEG analysis
For 10uV input & 320x gain  3.2mV / 800mV = -48dB VFS
 -48dB VFS + 68dB = 20dB SNRADC (marginal!!)
ADC Calculation (cont.)

Temperature sensor (志峻)


Based on the textbook (Precision Temperature Sensors in CMOS
Technology), a bipolar architecture below generates 2mV/ ⁰C
sensitivity (VBE)
For 2x DC (VGA) gain, we get full range swing (-20~60 ⁰C)
80 ⁰C x 2mV x 2 = 320mV < 1V (OK)
Resolution: 9-bit ADC 1V = 0.33 ⁰C / step < 0.5 ⁰C
 Need at least 9-bit ENOB  12-bit ADC is good enough
Temperature Sensor
Summary (11/18)

Based on accelerometer sensitivity @ 1mV/g



Based on Temperature sensor sensitivity @ 2mV/ ⁰C


Internal sensor LNA gain = 20
VGA gain = 1, 2, 4, 8
VGA gain = 2
With 12-bit, 0.8V full scale ADC

Accelerometer can support



Temperature sensor can support





+/-2g with > 200 LSB / g resolution & +/-16g with > 40 LSB / g resolution
LNA+VGA gain up to 160x to improve +/-2g resolution to >256LSB/g
Better than 0.33 ⁰C resolution
Humidity/Pressure (執中) internal sensors  direct 12-bit ADC conversion
may provide 8~9bit resolution within the sensing range
ECG (富彥/居正) external sensors support with 12-bit ADC requires
maximum 320x to achieve target SNR for 0.1mV small ECG signal
EEG signal can be as small as 10uV  320x gain stage + 12-bit ADC
provides only marginal performance. Need further investigation
ADC sampling rate @ max 10kHz


shall be capable of polling 8 sensors at >=1kHz sampling rate
However, LPF for SNR improvement cannot be performed if shared LNA