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ADPD 1080 Background ADC Value Is Larger Than LED Value

Category: Software

Hi,

I'm using the ADPD 1080 on own custom board, but utilize the microcontroller board that the interfaces with the wave tool software. For my application, I'm trying to make a measurement of oxygenation across a tube of blood (hence a transmission measurement) using two LEDs (one red one IR) and a large pin photodiode (1cm^2). I have gotten the board to communicate with the software and have attempting the make measurements of the light intensity for the leds and the configured the software such that each LED is in its own time slot. When I try to make a background measurement (no LED light), I keep seeing that the ADC output is larger than when the LEDs are turned on. I thought it could be a saturation but the problem still occurs even when I place the board in a black box. Is there an issue with my register settings below? Any help would be appreciated.

Thank you,

Osama

<<Addr>>	Value
<00>		1800
<01>		C0FF
<02>		0005
<03>		0000
<04>		0000
<05>		0000
<06>		0F00
<07>		0060
<08>		0A16
<09>		00C8
<0A>		0000
<0B>		0000
<0C>		0000
<0D>		0000
<0E>		0000
<0F>		0000
<10>		0000
<11>		3011
<12>		0050
<13>		0320
<14>		0559
<15>		0110
<16>		3000
<17>		0000
<18>		0000
<19>		0000
<1A>		0000
<1B>		0000
<1C>		3000
<1D>		0000
<1E>		0000
<1F>		0000
<20>		0000
<21>		0000
<22>		1030
<23>		3030
<24>		103F
<25>		67DF
<26>		0000
<27>		0700
<28>		0000
<29>		4000
<2A>		0000
<2B>		0000
<2C>		0000
<2D>		0000
<2E>		0000
<2F>		0000
<30>		0222
<31>		011E
<32>		0320
<33>		0818
<34>		0200
<35>		0222
<36>		011E
<37>		0000
<38>		0000
<39>		1310
<3A>		22FC
<3B>		1310
<3C>		3006
<3D>		0000
<3E>		4320
<3F>		4320
<40>		1010
<41>		004C
<42>		1C34
<43>		ADA5
<44>		1C34
<45>		ADA5
<46>		0000
<47>		0080
<48>		0000
<49>		0000
<4A>		0000
<4B>		2695
<4C>		0004
<4D>		0098
<4E>		7040
<4F>		2090
<50>		0000
<51>		0000
<52>		0040
<53>		E400
<54>		0AA0
<55>		0000
<56>		0000
<57>		0000
<58>		0000
<59>		0808
<5A>		0010
<5B>		0000
<5C>		0000
<5D>		0000
<5E>		0808
<5F>		0001
<61>		0000
<62>		0000
<63>		0000
<64>		0000
<65>		0000
<66>		0000
<67>		0000
<68>		0000
<69>		0000
<6A>		0000
<6B>		0000
<6C>		0000
<6D>		0000
<6E>		0000
<6F>		0000
<70>		0000
<71>		0000
<72>		0000
<73>		0000
<74>		0000
<75>		0000
<76>		0000
<77>		0000
<78>		0000
<79>		0000
<7A>		0000
<7B>		0000
<7C>		0000
<7D>		0000
<7E>		0000
<7F>		0000
  • The offsets in 0x17 to 0x21 are set zero so the output will be mid-scale.  Zero codes should be around 2048 codes.  The exact value varies from part to part a bit.  One of the things that can happen if the light is too-bright is a situation called over saturation.  In this scenario there is too much charger for the TIA to sink from the PD so the electrical pulse is extended past when the light has turned off.  Nominally the dark reference step happens in this phase past where the light has turned off.  When signal extends into this phase it can cause a signal to decrease with increasing light.  1) try a lower current setting 2) drop the TIA gain to the lowest setting to see if this changes the result.  3) sweep the AFE Offset value to ensure that the sampling region is in the optimal location.