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TL084: TL084 zero-input and low temperature/ high humidity issues

Yang Wu
Expert 3140 points
Part Number: TL084

Hi Team,

My air conditioner customer has some problems about the TL084 current acquisition circuit. (Circuit below and attached tina files)

The +/- 8V (maximum range) sin signal @60Hz comes from the Hall sensor, then it is inputted to the inverting port of the amp through the divider network (R1, R2,R3). The gain is set at G=2. normally the output should vary around 0 level, thus +/- 3.46V (maximum range) sin signal @60Hz.

Finally, the +/- output signal will be translated to 0 ~ 3.3V signal, which is compatible with the MCU sampling port. 

Now customer has 2 problems:

1. zero-input drift. when there has no current on the line, hall sensor has zero voltage and output should have zero voltage, but the output signal of the amp has voltage amplitude higher than 0.26V (threshold set by the MCU), triggering FAULT message.

2. Low temperature and high humidity working environment  (condensate water around TL084): it still has no current on the line, hall sensor has zero voltage and output should have zero voltage, but the output signal of the amp has voltage amplitude @ 3.1V, triggering FAULT message.

These 2 problems are not easy to re-occur and customer are going to MP at the beginning of next month, could you help analysis? 

Thank in advance and best regards,

Yang WU

TL084 current acquisition circuit.TSCTL084 Stability analysis.TSC

  • 0
    TI__Guru**** 157036 points

    Hello Wang,

    Before addressing this issue (in my next reply),  I noticed that this circuit is far more complex than needed. The DC transfer function could be accomplished with just 3 resistors. Convert +/-8V to -0.08V to 3.38V

    https://e2e.ti.com/blogs_/b/analogwire/archive/2016/09/20/three-ways-to-scale-an-analog-input-signal

    In this case resistor A = 10k, B = 7621, C = 4325 ohms. V = 3.3V Round B and C to nearest 1% resistor or select a pair of 1% resistors that get closer to the desired value. 

    For example B=7621 could be 14k || 8060  and C=4325 could be 16.2k || 5.9k  (5 resistor total)

  • 0 in reply to Ron Michallick
    TI__Guru**** 157036 points
    Wang,

    Phase margin can be easily increased to 65 degrees, a much safer value. I removed (set to zero) C1, C3 and R5. I set C2 to 680pF to match performance of C4.

    The DC output shift due to op amp input offset error is 2.5 times Vio. Some TL084 version could have a Vio as high +/-15mV ; thats a +/-38mV output range. This shift is sample to sample, not necessarily over time on a single sample.

    For problem 1, are you saying there is an AC signal present with no input?
    For problem 2, I don't see how final output could be 3.1V , unless maybe the op amp loop was oscillating?

    I understand that the symptom is rare, but it will be necessary to measure other test points in the circuit to find out where the problem originates. Perhaps the component change will solve the issue.

    How much signal bandwidth do you require?
  • 0 in reply to Ron Michallick
    TI__Expert 3140 points

    Hi Ron,

    thanks for your explanation and solutions.

    This circuit is inherited from the old air conditioner circuit and now used in the new commercial air conditioner project (though it's very complex...). The current acquisition is located at the 380V phase. the sampling frequency won't be high, supposing 1KHz is enough. 

    Considering customer cannot easily re-build this issue and MP urgency, it's difficult to identify the problem. As you said, I have been suggested customer to increase the phase margin.

    thanks,

    Yang