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ADS1291/2 in Pulse Oximeter Application

jogi loew
Intellectual 290 points
Other Parts Discussed in Thread: ADS1292

hi,

i'm trying to make one channel of the ADS1292 work as the analog input of a pulse oximeter. the photodiode is connected to a trans impedance amp and it's output to the positive input of the converter. negative is connected to ground. 

everything works kind of fine but it seems that i got a timing problem. my led driving circuit is synchronized with the nDrdy output of the ADS1292 (fs = 1khz to take 1 sample each ms, the led driving signals are according to p6, http://www.ti.com/lit/an/sprab37a/sprab37a.pdf).

When receiving a nDrdy interrupt i reset the H-bridge and collect the ADC data via SPI. After 550us i switch on the led (R/IR alternating) and wait for the converter do finish (which is after 450us when nDrdy goes low again).

Is there a way i can better synchronize the sampling time of the analog signal by the ADC to the falling edges of the R/IR led drive signals? Right now i got good looking waveforms but when I don't switch on one of the lights (doesn't matter which one) i still receive a signal, which is a bit weird...

Thanks very much,

J

  • 0
    Intellectual 290 points

    by changing the sampling rate to 8khz and only using every 8th sample i got the sampling period of the ADC limited to max 1/8ms before the falling edge of each "useful" nDrdy interrupt. I get 2 clean signals now.

    for my battery powered application it's not the way to go, but for now it will do the job...

    can anybody point me to a timing diagram showing "Analog Input Sample until nDrdy Falling Edge" timings... couldn't find it in the datasheet?

    thanks and regards, j