This is a comprehensive driver allowing to change most configuration settings of the Pulse Oximeter Analog Front End from Texas Instruments.
It is expected that the
- SPI interface is wired correctly.
- Chip select line is wired
- Dataready signal is wired
- Power Down signal is wired
Other signals are optional.
- Photo Diode Alarm
- LED Alarm
- Diagnostics End
Functions are privided to:
- Set the Photo Diode Amplifier settings
- Set the Transmitter LED current and voltage
- Set the digital IO
- Set the measurement timing
Currently the Heart Rate and Oxygenation algorihm is not implemented. A layout how those algorithms will work once implemented is shown.
For a hardware imlementation using this driver check out the SPO2_Board at Biomedical Sensor Board.
For explanation on measuring artial blood oxygenation, read Pulseoximetry
Urs Utzinger, May/June 2024
A program to change the AFE settings was developed. It takes the following commands. For an explanation look into the descriptions further below.
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| AFE4490 Texas Instruments Analog Front End for Clinical 2CH Pulse Oximeter |
| 2024 Urs Utzinger |
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| ?: help screen | r: dump registers |
| z: print sensor data | t: timing factor t1.0 |
| d: run diagnostics | p: alarm pins p0 |
| a: apply settings | s: show settings |
| .: toggle start/stop DAQ | e: show timers |
| m: set averages m4 | |
==Receiver==============================|==Transmitter==========================
| Rg1: stage2 gain LED1, Rg1-1 | TV: ref voltage level, TV0 |
| Rg2: stage2 gain LED2, Rg23 | TC: current range, TC0 |
| RC1: feedback capacitor LED1, RC15 |======================================|
| RR1: feeback resistor LED1, RR1200 | LP1: power level LED 1, LP1255 |
| RFC: filter edge frequency, RFC0 | LP2: power level LED 2, LP2255 |
| RFB: ambient Current, RFB1 |======================================|
| RC2: feedback capacitor LED2, RC25 | |
| RR2: feeback resistor LED2, RR2200 | |
==Toggle================================|==Toggle===============================
| x: 1 sameGain LED1&2 on/off x1 | x: 5 RX on/off x5 |
| x: 2 tri state on/off x2 | x: 6 TX on/off x6 |
| x: 3 h bridge on/off x3 | x: 7 AFE on/off x7 |
| x: 4 bypass ADC on/off x4 | |
==Debug Level===========================|==Debug Level==========================
| l: 0 none l0 | |
| l: 1 errors only l1 | |
| l: 2 warnings l2 | |
| l: 3 info l3 | |
| l: 4 debug l4 | l: 99 continous data reporting |
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The AFE44XX is typical implementation of two channel pulse oximeter driver and receiver for clinical transmission sensors. These are not typically used in smart watches but standard in the operating room.
The receiver is a classical transimpedance amplifier with programmable gain and low pass filter. In addition, offset currents and a second stage amplification can be programmed.
The transimpedance amplifier feedback resister
Capacitor value from 0..275pF can be selected. 5pF is the default. The capacitance is created through a combination of 5 capacitors: 5, 15, 25, 50, 150 pF. When setting the capacitance, the software will choose a combinatin of them to achieve closest possible value. Lowering the value increases noise and fidelity of the signal.
Resistor values of 0, 10, 25, 50, 100, 250, 500 and 1000 kOhm can be selected. Only one resistor can be selected at a time. 500 kOhm is the default. Increasing the value increases the gain and increases the noise. It can be assumed that the default value is optimal for noise.
| Value | Gain |
|---|---|
| 0 | 1 |
| 1 | 1.5 |
| 2 | 2 |
| 3 | 3 |
| 4 | 4 |
It is better to increase the gain of the transimpedance amplifier compared to increasing the second stage gain as in general the gain of the first stage should be opimized before adding gain on the other stages. If the signal is to low, one should increase the LED power.
A low pass filter is used before the DAC to prevent aliasing. The standard setting 500 Hz should be chosen. When increaseing the sampling rate by shortening the measurment timing, a higher frequency might be appropriate.
| Value | Filter |
|---|---|
| 0 | 500 Hz |
| 1 | 1000 Hz |
A background of 0, 1, 2, 3, 4, 5, 6, 7, 8, 9 micro Amp can be subtracted from the detector. When light is blocked to the photo diode, its reading should be zero. However there usually is a small offset which can be subracted. Since for SPO2 calcualtion the maximum and the minimum of the signal is needed, the signal should be background free.
Most implementations will want to use the internal H bridge to drive the LEDs.
AFE4400 has two settings for the current range (0,1) for 0 and 50mA. AFE4490 has three settings for current range (0/2,1,3) and the current through the LEDs is depending on the Voltage applied to the driver.
| voltage range | 1 | 0/2 | 3 | ||
|---|---|---|---|---|---|
| Voltage | 0.5 | 0.75 | 1 | V | |
| current range | 1 | 50 | 75 | 100 | milliAmp |
| current range | 0/2 | 100 | 150 | 200 | milliAmp |
| current range | 3 | 0 | 0 | 0 | milliAmp |
While common SPO2 probes use LEDs that operate between 30 and 50mA, they can be pulsed and the instantanous current can be larger than the average current.
After setting the power range, the LED power can be set for each LED to a value of 0..255 which is proportional to the max current delivered to the LED as shown above.
Since pigmentation, age and weight affect the transmission of the light through tissue, one should increase the LED power for each subject so that most bits of the ADC converter are utilized. The ADC produces 22bit 2's complement which is + 2,097,151 and - −2,097,152. Therfore the signal power should be increased so that the signal is about 1,000,000.
For SPO2 calculations a ratio of the peak and value in the optical signal is utilized and the signal does not need to be "proportional" between the two LED wavelengths. Therefore the program should be opmized for SNR at both LEDs and each LED can have different power settings.
A timing sequences is set that for each loop a reading at LED1 and LED2 is available as well as the corresponding ambient light levels. Ambient light level is subtracted from the readings at LED1 and LED2 in the AFE.
A stanard timing sequence from the datasheet is used and can be modified with a factor. If factor is 1.0, the default values are used. This results in a measurement sequence executing 500 times per second. For other Factor values, timing values are multplied with Factor but the reset pulse length and sampling offset is not altered.
PRPCOUNT defines the length of a measurement sequence and needs to be between 800 to 64000. One can assume that a longer sequence results in longer sampling and therefore less noise.
The measurement process is:
- Turn LED on
- Sample Signal
- Turn LED off
- Reset ADC
- ADC Conversion
To measure ambient, LED1 and LED2 a fixed sequence is used:
- Ambient LED2 (LED1&2 off)
- LED1
- Ambient LED1 (LED1&2 off)
- LED2
- Repeat
The default timing for
| On | Sample | Convert | ADC Reset | |
|---|---|---|---|---|
| LED2 | 6000-7999 | 6050-7998 | 4-1999 | 0-3 |
| LED1 | 2000-3999 | 2050-3998 | 4004-5999 | 4000-4003 |
| Ambient LED2 | 50-1998 | 2004-3999 | 2000-2003 | |
| Ambient LED1 | 4050-5998 | 6004-7999 | 6000-6003 |
This results in LED pulses of 0.5ms and 500 background subtracted measuarements for both channels per second.