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TIDA-00316: Question for 4.3 section selection of OP-AMP

Xiaobing (Eric)
Intellectual 895 points
Part Number: TIDA-00316

Hi All,

  Thanks for your times, I have a question when i learnt this documents, i noted 4.3 Selection of Op-Amps, how to understand each of items? especially  for item 1. Thank you. 

  REQUIREMENT, BENEFIT

1. High input impedance, Minimizing this reduces input loading on sensor and minimizes input current offsets on High input impedance input resistors.

2. Input current noise, Minimizing this reduces the amount of current noise that becomes converted to voltage Input current noise noise on input resistors. Voltage noise

3. Voltage noise, Minimizing this improves the overall signal-to-noise ratio. Maximizing this reduces the amount of input offset changes due to high dv/dt at the CMRR versus frequency inverter output.

4. CMRR versus frequency, Minimizing this reduces the amount that the total unadjusted error changes at the output Voltage offset drift of the op-amp.

5. Voltage offset drift, Designing a single-supply amplifier simplifies the system supply requirements; this Single supply or dual supply operation usually correlates with lower power architecture. However, if the signal coming from sensor itself is bipolar, it is good to use op-amp with dual supply options.

6. Input type, Rail-to-rail input can help to interface sensors having large input swings.

7. Output type, With rail-to-rail output configuration, the amplified input signal can reach the supply Output type voltage and can use the FS range of ADC in a better way

  • 0
    TI__Mastermind 26620 points
    Hello,

    Please consider using TI's Precision Labs training videos as they completely explain basically all of these requirements in easy-to-follow videos. You can find them here:

    www.ti.com/precisionlabs

    ---------

    Anyway,

    1. High input impedance of the op amp is important because it does not load the sensor or signal you are connecting the op amp to. In Section 4.3 the wording might be confusing because it says "Minimizing this reduces input loading on sensors and ...", but in reality you want your op amp's inputs to have the most maximum input impedance possible (note this is the case for voltage-feedback op amps and not current-feedback).

    2. You may want to choose an op amp that has specified a low input current noise in the datasheet. Minimizing this noise is important because it will be converted from current to voltage because of your input resistors and feedback resistors and this will add noise to the output signal (Vout) of your op amp.

    3. Op amps also specify voltage noise (separate from current noise). You minimize this for same reason as 2.

    4. Having a good CMRR versus frequency is important if you want your op amp's internal voltage offset (Vos) to not vary much with input common mode voltage (Vcm) of your amp. This varies with frequency of input signal.

    5. Temperature inherently changes the internal voltage offset (Vos) of an op amp. This Vos is gained up and seen as error on your output and thus will get worse with temperature if the Vos_drift is high.

    6. 7. You always need to consider if the signal your inputting into an op amp fits within the acceptable common-mode (Vcm) input signal range (specified in datasheet). Additionally, you need to consider if the signal you want to see on the output (Vout) of your op amp fits within the acceptable Vout range of the op amp in its specific configuration.

    Peter Iliya
    Precision Amps Tucson
  • 0 in reply to Peter Iliya
    Intellectual 895 points

    Hi Peter,

     Thanks for your time and your comments. Yeah, you're right, when I learnt item 1 descriptions, which make me confusion. Thanks for your confirm. Nice day.

    Regards,

    Xiaobing (Eric)