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about adc141s628 on lmp91200eval

YUSHU CHEN
Prodigy 200 points
Other Parts Discussed in Thread: LMP91200, ADC141S626

hi sir,

i use lmp91200eval to measure water ph with a ph probe.

i encouter a issue when reading adc141s628.

that is i can't read negative value from adc141s628. the lowest value of adc141s628 is 0,no negative value is got.

when i calibrate with ph4 buffer,values read from adc141s628 are positive,and these are correct.(i think so.)

when i calibrate with ph7 buffer, values read from adc141s628 are around zero,but never under zero.

when i calibrate with ph10 buffer,values read from adc141s628 are all zeros.(<- this is wrong,invalid.)

i read spec of lmp91200,adc141s626,LMP91200EVAL_REV0p7, and try to make it right,but i failed.>"<

Could you give me some suggestion about this situation?

bty,all paramters follow spec. of lmp91200.pdf,page 21 Table1 Configration register of PH measurement

i check voltage of several points,

LMP91200 vdd=3.3v,VREF=2.5v, VOCM=1.25,VOUT~1.25V

adc141s628 VA=5v, VIO=3.3v, VREF=2.5v,

VOUT-VCOM= 0.15v as ph4 buffer is used.

VOUD-VCOM= -0.15V as ph10 buffer is used.

  • 0
    TI__Mastermind 23425 points
    The ADC141S628 is a pseudo-differential ADC and thus only performs conversions no positive inputs. the inverting terminal must be kept at a common mode voltage very close to 0V (ground). the non-inverting input gets converted relative to this ground input. the internal architecture of the device is still differential but the input can only be positive. So output codes of 0 to 16383 are for input of 0V to +VREF. the ADC141S626 is a differential ADC where the input dynamic range is +VREF to -VREF. due to the dynamic range being twice as large (2xVREF), the lsb size for a differential ADC is twice that of the pseudo-differential ADC.
    Chuck