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OPA378: OPA378 for high source resistance thermopile

ravi kumar89
Prodigy 80 points
Part Number: OPA378
Other Parts Discussed in Thread: LMP7731, OPA325, OPA376

Hi,

I am planning to use OPA378 for interfacing a thermopile that has 68kOhm series resistance. circuit attached. On the non-inverting input,  i have the thermopile.

1. Due to the imbalance in the series resistors on inverting and non-inverting terminals of the opamp, will this circuit cause any errors? (due to bias currents and auto zero feature)

2. adding a capacitor to Gnd on the non-inverting terminal will help in filtering the input?

3. due to the auto calibration at 350kHz, it is mentioned in datasheet that " no aliasing or flicker noise". i believe this is about the 1/f noise at very low frequencies. but that is the effect of this auto calibration on the output? need recommendation on the output filtering.

  • 0
    TI__Guru* 93105 points

    Hello Ravi,

    The OPA378 uses a chopper op amp design. Three things that assure the best performance from a chopper op amp are low source impedance, matched impedance paths for each of the inputs, and low to moderate voltage gain. Observing those points helps reduce the conversion of current noise to voltage noise associated with the charge transfer of the input chopping switches. High gain amplifies amplify such noise by that high gain factor. Although your OPA378 circuit will function in your voltaic pile application, the performance could be reduced because of the high source impedance, input path impedances and voltage gain selected for your circuit.

    Since this is essentially a dc circuit it probably doesn't require a lot of bandwidth. Adding capacitors across the source and the feedback resistance would reduce broadband noise and enhanced chopping noise, but determining the which capacitor values to use would come down to setting up the circuit, observing the output noise and trying different values. If you find you can get satisfactory performance, then indeed a chopper will have the best noise at dc and very low frequencies due to the absence of 1/f noise.

    You may find that a low-noise, non-chopper op amp provides very satisfactory low frequency noise performance. Bipolar input op amps have the lowest noise of the op amp technologies. Your application shows a low supply voltage so that does limit the number of available op amps. The LMP7731 is an option for a low-noise, low voltage bipolar op amp that might work well in the circuit:

    http://www.ti.com/lit/ds/symlink/lmp7731.pdf

    Let me know what you think of this option.

    Regards, Thomas

    Precision Amplifiers Applications Engineering

  • 0 in reply to Thomas Kuehl
    Prodigy 80 points

    Hello Thomas,

    Thanks for a nice explanation of the tradeoffs. If you ignore the high series resistance on the input of my circuit, can u tell the frequencies of signals i can see due to the chopping action? any test results with this opamp will also help me. Yes i planned for a capacitor across the feedback, on non-inverting input, and RC filter on optput of opamp to my ADC.

    Coming to the non-chopper based opamps, i noticed that the contribution of high current noise in bipolar opamps is contributing more the output noise than the input voltage noise. in LMP7731, the input current noise is high compared to OPA378. This will multiply with the series resistance of sensor and the gain as per my understanding.

    Do you suggest a CMOS based opamp with low offset voltage for my application? voltage output from the thermopile is in the order of few mVs at maximum.

    Regards

    Ravi

  • 0
    Guru 140200 points

    Hi Ravi,

    can you specify the thermopile? Datasheet?

    Kai

  • 0 in reply to kai klaas69
    Prodigy 80 points

    HMS-Z11-F5.5 is the thermopile.

    While searching for opamps with small size, atleast 2.5V operation, i found OPA325 another suitable one. Offset voltage is a bit higher than OPA378.

    OPA325 is not Zero- drift but i couldn't see how you are achieving low offset voltage.

  • 0 in reply to ravi kumar89
    TI__Guru* 93105 points

    Hi Ravi,

    The OPA325 CMOS op amp uses TI's proprietary e-trim(TM) electronic trim technology to reduce the voltage offset to very low levels, 40 uV typical at TA = 25°C. It does not rely on chopping, or auto zero techniques to achieve the very low offset. Therefore, no switching artifacts to deal with. Its Input voltage noise density is typically 10 nV/√Hz at 1 kHz. The OPA325 in the same family and has a typical input voltage noise density of 8.5 nV/√Hz at f = 1 kHz.

    https://www.ti.com/lit/ds/symlink/opa320.pdf

    The OPA376, which uses e-trim(TM) as well and is rated with a 5 uV typical voltage offset. It has a typical 1 kHz noise of 7.5 nV/√Hz at TA = 25°C.

    https://www.ti.com/lit/ds/symlink/opa376.pdf

    They are CMOS, extremely high input impedance, low input current op amps so their input offset due to Ib and current noise will be less affected by higher source and path impedances.

    Regards, Thomas

    Precision Amplifiers Applications Engineering