• State Resolved
  • Locked Locked
  • Replies 4 replies
  • Subscribers 49 subscribers
  • Views 969 views
  • Users 0 members are here
Support feedback
Options
Options
Similar topics

This thread has been locked.

If you have a related question, please click the "Ask a related question" button in the top right corner. The newly created question will be automatically linked to this question.

TLV522: TLV522 saturates at 1.7V when rail is 5V

Nikhil Bapat
Intellectual 860 points
Part Number: TLV522
Other Parts Discussed in Thread: LP2950,

I have been working on a circuit to interface a sensor with MSP432P401M. The sensor gives output from 0.5V to 4.5V and has frequency response of 19 Hz based on the specifications. I am seeing a weirdest problem here. The Opamp is saturating at 1.7 volts.The Opamp give proportional output until 1.6 or 1.7 volts but then does not go beyond that. I have replaced the Opamp and the resistors around it. I have checked if it is getting the correct supply and input and all seems to be in order. And the most baffling part was that it was working for couple of weeks before this behaviour suddenly started.

  • 0
    TI__Expert 3555 points

    Nikhil,

    This is an interesting circuit. I notice that the output of U5-B is not connected to R7, is that correct in your circuit?

    From my analysis the voltages should be (if R7 is connected to the output of U5-B)

         Your supply voltage on the op amp is 5V

         sensor out = 4.5 V, U5-A output = 1.928 V, U5-B output =3.375 V

         sensor out = 0.5 V, U5-A output = 0.214 V, U5-B output =0.161 V

    What are the values that you are getting?

  • 0 in reply to Bonnie Baker - WEBENCH Design Center
    Intellectual 860 points

    Bonnie,

    R7 is connected to the output node of the OPAMP to complete the feedback loop, sorry about the incomplete sketch.

    According to my understanding, the first opamp (U5-A) will be voltage follower, so it's output will be same as sensor output. The second opamp (U5-B) will bring the voltage level down to ADC voltage levels. For me the calculations were

    Sensor Out 4.5V, U5-A Out 4.5 V and U5-B Out 3.375 V

    Sensor Out 0.5V U5-B Out 0.5 V and U5-B Out 0.375 V

    The output at U5-B is saturating at 1.7 volts only. The very first time after the circuit was assembled I ran it through a quick run to check if everything was in order and this is what I got:

    Sensor Out V                         ADC In V (i.e. U5-B Out)

    0.53                                          0.3975

    1.012                                        0.759

    2.032                                        1.524

    3.032                                        2.274

    But now the OPAMP output does not go beyond 1.7 V. Since all components are SMD, I could not check how much current was being drawn by the opamp. Is it possible that opamp is drawing more current than the LP2950 can supply?

  • 0 in reply to Nikhil Bapat
    TI__Mastermind 23050 points

    Hi Nikhil,

    I'd like to clarify a few things about your setup:

    1) Is this circuit being tested by itself as shown above or is it actually driving the ADC? It is not likely that the TLV522 needs to draw more current than the LP2950 can supply, but it is possible that you are exceeding the output drive capability of the opamp, which is only a few milliamps. 

    2) What kind of sensor are you attempting to interface with the ADC? 

    3) Do you have any oscilloscope shots you can share?

    Regards,

    Zak Kaye

    Precision Amplifiers Applications

  • 0 in reply to Zak Kaye
    Intellectual 860 points
    Zack and Bonnie,

    I was able to pin point the issue yesterday.
    The opamp circuit above is interfaced with the ADC of MSP432P401M microcontroller on a 4 layer PCB. One of the reasons we chose TLV522 was that it can give rail to rail output and ADC does not need very high current. The whole board is assembled. The sensor we are interfacing is a capacitance based tilt sensor which has its own circuit which will give the output in DC form.

    The reason this was happening was because the ADC pin was damaged. We were checking the connectivity of all components on the PCB and the opamp out to ground showed only 30 ohm resistance. So we disconnected the ADC pin by lifting it up and the opamp out to ground showed open connection on a DMM. So after that I checked the ADC pin to ground connectivity and it shows 30ohm resistance. This is all in complete power off state. So I think something damaged the ADC and so it was drawing too much current which was loading the OPAMP.

    Now we are cutting the track and soldering the opamp output to ADC of the second bank by external copper wire. This will do for testing purposes but if the issue repeats then we will need to work on it.