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lmp90100 can read and write but ADC values changes

rameshwar kumbhar
Prodigy 120 points
Other Parts Discussed in Thread: LMP90100

My testing of lmp 90100 is at last stage i can read/ write lmp90100 but when i read the adc values from registers 0x1A,0x1B,0x1C with fix input voltage then the value of register 0x1A remains stable but the values of register 0x1B and 0x1C are not stable they changes every read.

Consider if i give the fix input voltage of 0.076mV  then if i read the values from 0x1A, 0x1B, 0x1C then the output is ok i.e. output get as 0.0758,but when i read again the values of register 0x1B, 0x1C changes and output gets as 0.0757.

thus every read the output varies as randomly some time incrases and next time decreases again increase such that  0.0758mV, 0.0757mV, 0.0759mV, 0.0758mV.

Anyone can solve my problem?

i configured lmp90100 in background calibration mode 2

i configured registers as given 

Adr 0x00 RESET
Val 0xC3
// =====
Adr 0x01 //spi handshake control
Val 0x04
// =====
Adr 0x03 //spi stream control
Val 0x01
// =====
Adr 0x08 //power mode control and status
Val 0x00
// =====
Adr 0x10 //background calibration control
Val 0x02
// =====
Adr 0x11 //spi data ready bar control
Val 0x83
// =====
Adr 0x12 //adc auxillary control
Val 0x27
// =====
Adr 0x14 //sensor diagnostic threshold 1,0 (0x14 --- 0x15)
Val 0x00
// =====
Adr 0x15
Val 0x00
// =====
Adr 0x1D // crc data
Val 0x00
// =====
Adr 0x1F //channel scan mode
Val 0x80
// =====
Adr 0x20 //channel input control
Val 0x41
// =====
Adr 0x21 // Gain:1 - ODR:214
Val 0x11

i am reading values from ADC out registers one at a time.

if i am going wrong please help.

Thanks in Advance

  • TI__Genius 16890 points

    Hi,

    You are getting very good results.  Your power supplies are most likely the cause of the variation.  You would need absolutely stable supplies for VA, Vref, and the input to get a result that doesn't have some variation.  Lab supplies will not do that.

    Mike

  • in reply to Michael Stout
    Prodigy 120 points

    Hi, Michael Stout

    Thanks for your valuable support.

    So i concentrate on my power supplies.

    I am giving same supply of 3.3v to both VA and VIO and the capacitors are 0.1uF ceramic and 1uF electrolytic

    Is this cause to variations?

  • in reply to rameshwar kumbhar
    TI__Genius 16890 points

    Hi,

    Yes, if there are variations in the Vref supply this would cause variations in your output.  VIO is a digital supply and will not influence the output.  VA can cause variations in the output, especially if it is also used to bias the inputs.

    Mike

  • Prodigy 20 points

    I also want to contribute to this forum by sharing my doubt. For single channel there is no issue with ADC reading.

    Consider the case , If I configure (suppose)  4 single ended channels and ch_scan_reg is configured for multi channnel continous scan. In this case do i need to read address 0x1a , 0x1b and ox1c  4 times ?

    pls share more info regarding multi channel data taking from ADC.

    Thanks

    Aki