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INA126: Help selecting an amplified for pyranometer

Part Number: INA126
Other Parts Discussed in Thread: INA122, , ADS1115, LM7705, OPA320

I've got a 2 wire pyranometer. It generates voltage 0-40mV based on the strength of the solar irradiation.

I've got in my toolbox INA126 and INA122.

Are they the right devices to boost the 0-40mV output?

If I understood the data sheets, the inputs can be from -0.7V to 0.7V, is that correct?

Kind regards

Dean

  • Hi Dean,

    To evaluate whether these INAs are a good fit for this application we would need to know more about what you are after. For example:
    1) What is your required output range and what are your available supplies?
    2) What are you driving with the INA?
    3) What kind of accuracy do you need from this measurement?
    4) What does the sensor look like? Is this a differential signal?

    I'm not sure where the -0.7V and 0.7V came from, the valid input range depends on the supplies, the gain, and the reference voltage.
  • Hi Zak,

    Thanks for your response.

    1. Say 0-3.3 or 0-5V output will be ideal.

    Power supply I’m flexible. If single supply is

    enough, I’ll derive say from raspberry pi board.

    If dual supply configuration is required, I’ll devise an alternative.

    2. Some ADC, say ADS1115 or something else suitable to ina, feel free to suggest

    3. Considering the pyranometer is quite expensive I’d like to read as close to the real value as possible. Each 18 microV corresponds to 1W/m2

    4. Yeah, the sensor is passive, takes no power and instead generates voltage at its 2 terminals. Input impedance is 40 ohm

    Regards

  • Hey Dean,

    I would recommend using a 5V output range as this buys you a little more resolution because you can increase the gain.

    It would be best for you to use dual supply, because this will allow you to get a true 0-5V output. On a single supply, the lowest signal you could measure with the INA122 would be 800uV, and you would also need to bias the input to a common-mode level within your supplies to get the full output swing. One thing you could consider doing would be to use a charge pump for your negative rail, such as the lm7705. This device takes a 5V input and produces -230mV. This would give you enough room to get the full output swing and you wouldn't need to lift the inputs.

    If you're looking to get 16bit resolution on this measurement, you're also going to need a buffer amplifier between the INA and the ADC. The OPA320 would be a good candidate for this.

    Since your sensor is effectively a floating voltage source, you will also need to include resistors to ground on the input (or to some bias voltage if you decide to take that route). This is shown in figure 3 on the datasheet.
  • Thank you kindly,

    Attached is my understanding of your advice, albeit with INA126.

    I couldn't find LM7705 in Tina so I approximated it with a battery source.

    My only issue now is the expected output voltage is not in tune with the gain.

    Can you shed some light on it please?

    Kind regards

  • Dean,

    Sorry for the confusion, the recommendation I made above was specific to the INA122. I chose this device because it has more relaxed input/output limitations. The INA126 would require a larger negative supply of at least -1.5V.