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Query regarding LMP91000 external and internal gain settings for TIA amplifier

Rohit6230
Intellectual 745 points
Other Parts Discussed in Thread: LMP91000

I have Query regarding  LMP 9100 - TIA control register, currently I’m working on  2 kinds of 3-Lead Amperometric Sensor. One is Biased Sensor & other one is Unbiased Sensor.

For Biased Sensor I’m connecting an External Resistor, eg: 25K (25K connected across pin C1 & C2), so my TIACN config : 0x01

For Un-Biased Sensor I’m TIA Feedback Resistor, eg: 35K, so my TIACN config : 0x15

 

Q’s:

 *NOTE*  In case of Un-Biased Sensor, I’m using TIA Feedback Resistor (35K), additionally I’ll be connecting 25k resistor.

              Since I’ll be not changing my circuit design for both type of sensors, as I’ll be switching between External resistor and Internal TIA Feedback Resistor in TIACN, for Un-Biased and Biased sensors.

                  

 > Will there be any effect of External Resistor to my internal TIA Feedback Resistor?

 > Since both RTIA (internal on-chip gain adjust) and External resistor will come into parallel combination, due to which will it affect the Output end result?

 

  • 0
    TI__Prodigy 420 points

    Hi Shravan,

    When an external resistance is connected to LMP91000, it will be always in parallel to the internal resistances (when they are selected).

    The apps engineer (Domenico Granozio) for LMP91000 has come up with the following rule -

    Unbiased gas sensor: TIA in external resistance configuration, 35Kohm between C1 and C2.

    Biased gas sensor : TIA with internal 120kOhm , (since the external 35K is in parallel to the 120K) the final result will be 27Kohm.

    If 27Kohm is too high, you can find a compromise according to the following table, the last line should be a good compromise.

     

    external resistance (Kohm)

    Internal resistance (Kohm)

    Requested gain (Kohm)

    Real gain (Kohm)

    Error (%)

    Requested gain (Kohm)

    Real gain (Kohm)

    Error (%)

    35

    120

    25

    27.1

    8.4

    35

    35

    0.0

    34.5

    120

    25

    26.8

    7.2

    35

    34.5

    -1.4

    34

    120

    25

    26.5

    6.0

    35

    34

    -2.9

    33.5

    120

    25

    26.2

    4.8

    35

    33.5

    -4.3

    Regards,

    Shrikrishna Srinivasan,

    Texas Instruments.