Mixed-Signal Processing Unit for Sensing Signal Processor

1. Mixed-Signal Processing Unit for Sensing Signal Processor NCTU TWT Lab 2011/11/18 (Update)

2. SSP Platform Architecture Sensing Signal Processor (SSP) RTC Sensor A RF Ext. Sensing Actuating Module Sensor B AFEuni uP Aux port USB Aux port Sensor C Storage Ext Storage Sensor D+AFE Power Module Sensor E+AFE uP: 8051/Open RISC RF: Bluetooth USB:USB 1.0

3. 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. Universal Int. sensor LNA 執中 Pressure 昱賢 居正 Humidity Filter PGA D-S ADC To MCU Differential Cap. sensing Gain Control Ext. Ch1 Universal Ext. sensor LNA 居正 Ext. Ch2 SPI To MCU Gain Control Gain / Filter / ADC Control Logic Ext. Ch3 CMRR >100dB Gain Control Ext. ChN Differential Volt/Cap. sensing

4. 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

5. 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

6. 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 • 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 BOSCH G-sensor (Used in mobile application)

7. 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!!)

8. ADC Calculation (cont.) • Temperature sensor (志峻) • Based on the textbook (Precision Temperature Sensors in CMOS Technology), a bipolararchitecture below generates 2mV/ ⁰C sensitivity (VBE) • For 2x DC (VGA) gain, we get full range swing (-20~60⁰C)80⁰C 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

9. Summary (11/18) • Based on accelerometer sensitivity @ 1mV/g • Internal sensor LNA gain = 20 • VGA gain = 1, 2, 4, 8 • Based on Temperature sensor sensitivity @ 2mV/ ⁰C • VGA gain = 2 • With 12-bit, 0.8V full scale ADC • Accelerometer 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 • Temperature sensor can support • 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