Simsense: 2nd generation sensor overview

The Simsense second-generation sensor has the same sensors as the first-generation device, but includes improvements and learnings from the first generation. We introduced some location changes and enhancements to internal circuitry, and addressed some critical issues, yielding better signal quality and reliability.

Simsense includes six types of sensors:

  • Electrocardiogram (ECG)
  • Photoplethysmogram (PPG)
  • Galvanic Skin Response (GSR)
  • Bio-Impedance (Bio-Z)
  • Accelerometer
  • Skin Temperature (thermometer)

Sensor overview & electrical characteristics

ECG

The Electrocardiogram (ECG) sensor measures the electrical activity of the heart. The electrodes need to be placed across the heart, or in practical terms, one on each side of the heart. A clinical lead 1 signal measures from the left wrist to the right wrist. A surrogate for this measure can be done using the index finger of the right hand touching the left wrist, with the electrodes in between. The position of electrodes on the band is less important. So long as the electrodes enable this connection of the left and right arms/wrists/hands.

Parameter Options Value Units
IA gain 24/68/220 24 V/V
PGA gain 1/2/4 1 V/V
ADC sampling rate 128/256/512 512 Hz
ADC resolution Fixed 18 bit (signed)
ADC ENOB Fixed 13.5 bit
FIR filter enable Yes/No Yes  
FIR filter bandwidth Depending on sampling rate 40 Hz

PPG

The Photoplethysmogram (PPG) sensor measures blood-volume changes in the blood tissue. PPG is a plethysmogram obtained using optics. A plethysmogram is volumetric measurement of an organ. In Simsense we use PPG to detect blood-volume change at microvascular level. This technique is non-invasive and it is obtained by illuminating light into the body and measuring the change in light absorption.

Simsense has multichannel PPG with multiple wavelengths. Table below covers PPG channels and their corresponding wavelength(as per LED data sheet):

Channel name LED color Wavelength(nm)
0/A Blue 460-470
1/B Green 520-535
2/C IR 850-970
3/D IR (SpO2) 850-970
4/E Green 520-535
5/F Green 520-535
6/G Red (SpO2) 644-676
7/H Green 520-535

Note: In monitor mode, PPG graphical UI color maps with the first generation module.

Parameter Options Value Units Notes
Resistor for TIA gain setting 50/100/200 50 KOhm(Ω)  
LED Pulse width 0 to 15µs (in theory) 3 µs In practice only 2/3/4µs makes sense
LED Pulse period 0 to 255 20 µs  
LED Pulse count 0 to 22 (in theory) 2x16=32   Will be optimized as trade-0ff SNR vs. power
LED current 10 to 250mA vary mA Due to ACC (Automatic current control)
ADC sampling rate 0.125 to 8192Hz (in theory) 50 +/- 5Hz Hz/LEDs Asynchronous clock, M0 interrupt handling will limit sample rate in practice
ADC resolution Fixed 14 bit  
ADC ENOB Determined by LED pulse count and ADC oversampling/decimator factor 14 + 0.5 log 2(LED pulse count) + 0.5 log 2(ADC oversampling factor) bit  
Automatic LED current control enable Yes/No Yes    

GSR

The Galvanic Skin Response (GSR) sensor measures conductivity of skin. The skin conductivity changes with both changes in the underlying amount of sweat released onto the skin from the sweat glands, and the number of active sweat glands.

Parameter Options Value Units Notes
Measurement range Fixed 0.0002 to 100 µSiemens  
Measurement range Fixed 50k to 5G Ohm(Ω)  
Applied DC differential voltage +- 0.1/0.9/3.7 +- 3.7 typ. V Will be controlled by automatic gain control
Transimpedance gain (tonic + phasic channels) fixed 100 KOhm(Ω) followed by 100x fixed second stage gain with digitally controlled feedback circuit
Channel bandwidth Fixed 0 to 4.8 Hz  

Bio-Z

For heart rate measurement, the bio-impedance sensor aims to measure the pulse wave of blood flow in an artery. This can be achieved because the blood has different impedance to the surrounding tissue, such that the variation in impedance as the volume of blood changes can be measured.

Parameter Options Value Units
Current frequency 2/20/40 40 KHz
Current amplitude Fixed 200 µA(peak-peak)
  Fixed 100 µA(RMS)
Current shape Fixed Square  
IA gain 70/140 70 V/V
PGA gain 1/1.2/2/3 1 V/V
ADC sampling rate Fixed 1024 Hz
ADC resolution Fixed 12 bit (unsigned)
ADC ENOB Fixed 11 bit
FIR filter enable Yes/No Yes  
FIR filter bandwidth Depending on sampling rate 80 Hz

Accelerometer

The accelerometer is an instrument which senses and measures tilt and motion.

Parameter Options Value Units Notes
ADC sampling rate 12.5/25/50/100/200/400 125 typ. Hz Asynchronous clock
ADC resolution Fixed 14 bit  
Measure Range +/-2/4/8/16 +/-8 g  
Worst case sensitivity error Fixed +/-10 % 6-position rest calibration (against gravity) could compensate sensitivity and offset errors if needed
Worst case offset Fixed +/-0.15 (X/Y), +/-0.25 (Z) g Same as above

Skin Temperature

The skin temperature sensor senses temperature of the skin enclosed by the sensor module.

Parameter Options Value Units Notes
Thermistor value Fixed 100 kOhm  
Thermistor value precision Fixed +/-1 %  
Thermistor Beta Fixed 4100 K  
Thermistor Beta precision Fixed +/-1 %  
Thermistor stabilization time Fixed Approx. 5 Minutes To reach thermal equilibrium after putting band on skin
Measurement range Fixed -40 to +85 oC  

Bias

The bias electrode is not a sensor channel in its own right, but it is a reference electrode for the electrical measurements ECG, GSR and BioZ. Its purpose is to maintain the correct DC bias potential between the human body and the electronics. This bias potential is approximately 0.6V with respect to system ground potential. The bias electrode needs good contact to the skin to ensure a good signal quality for the above mentioned measurements.

  • In normal mode, BIAS electrode will be 0.6V and GSR electrode 0.7V but these will be sporadically swapped so it can be the other way round
  • In GSR mode, the Bias / GSR (above Accel.) electrode is not used and one of the two GSR electrodes will be at 0.6V while the other will be at 0.7V, 1.5V or 4.3V

Simsense: Sensor Module Concept