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THS4551: Differential Input

Chiaki Tanaka
Prodigy 40 points
Part Number: THS4551
Other Parts Discussed in Thread: ADS127L01, ADS127L01EVM

To input the load cell output to the ADS127L01, I am using a THS4551 to amplify the differential signal. (I used the ADSL127L01EVM.)
The load cell specifications are a 5V power supply voltage, a rated output of 1mV/V, and a gain of 100x for the amplifier circuit. The ADC and op amp share a common ground, and the load cell output is input directly to the amplifier circuit without any level shifting.
When the load cell is powered at 5V, the differential voltage input to the ADC is approximately 3V, and the op amp output appears to be stuck to the rails. Compared to ground, AINP and AINN are approximately 2.5V. On the other hand, when the load cell is powered from the evaluation board's 3.3V power source, the differential voltage relative to ground is approximately 1.5V, and the system operates without any problems.
Regarding this phenomenon, is it correct to understand that the differential voltage input to the amplifier circuit does not need to be particularly concerned about the voltage relative to the ADC's ground? Also, I'm wondering why there's no problem when the voltage relative to GND is 1.5V, but the problem occurs when supplying 5V. Please check.
Thank you.

  • 0
    TI__Mastermind 18960 points

    Hi Chiaki,

    It seems like there is some kind of input common-mode voltage/output voltage violation which is why you see the outputs stuck. The input voltage range depends on the supplies at the THS4551 and ensure whatever input voltage is applied will not violate the input common-mode range. The differential voltage needs to be considered to ensure you are not gaining this difference beyond what the output is capable of outputting. Is there a circuit you can share that shows how this circuit is configured? From what you are sharing it seems like some kind of input/output violation but without looking at the circuit at these important nodes, it is hard to definitively say what is being violated.

    Best Regards,

    Ignacio

  • 0 in reply to Ignacio Vazquez Lam1
    Prodigy 40 points

    Hello, Ignacio

    I would like to confirm the following two points:

    1.Circuit Diagram
    Please find attached the circuit diagram based on the ADS127L01EVM. The modified resistor (R) and capacitor (C) values are indicated for your review.

    2.Input Common-Mode Voltage
    When using the THS4551 with a 3V single supply, could you kindly advise what range the input common-mode voltage should fall within?

    Thank you,
    Chiaki

  • +1 in reply to Chiaki Tanaka
    TI__Mastermind 18960 points

    Hi Chiaki,

    I believe you are likely running into an input common-mode voltage violation. Could you share the VOCM voltage when changing the load cell voltage and if this stays constant? The same goes for the supplies on the THS4551, is it constant at 3V? Just for my understanding, when you have 5V on the load cell each input (AINP and AINN) is 2.5V, and when the load cell changes to 3V each pin becomes 1.5V? For the input common mode voltage with 3V supplies, you would expect a device to function correctly with each input pin between -0.2V to 1.9V (typical spec at room temperature). I believe with 2.5V at the input, you are violating your common-mode voltage range. The reason it is maxing out at the output would depend on the value of VOCM as well. 

    Best Regards,

    Ignacio

  • 0 in reply to Ignacio Vazquez Lam1
    Prodigy 40 points

    Hi Ignacio,

    As you mentioned, when the load cell is driven with 5 V, the amplifier inputs (AINP/AINN) are around 2.5 V, and with a 3 V excitation they are around 1.5 V. Since 2.5 V exceeds the THS4551 input common‑mode range (−0.2 to 1.9 V at 3 V supply), I understand that this caused the output saturation.
    I will adjust the operating conditions so that the amplifier inputs stay within the allowed −0.2 to 1.9 V common‑mode voltage range going forward.
    Thank you,
    Chiaki