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Operating condition bellow 5V

Kazuyuki Miyashita
Prodigy 20 points
Part Number: OPA2227
Other Parts Discussed in Thread: OPA2277, OPA186, OPA277, OPA188, OPA325

Tool/software:

Hi, professors.

I'm using OPA2227 (OPAx227 series) at single power supply condition. The supply voltage is about +5V to 0V.

Recently, I noticed the output seems unsteady when the power supply voltage is below +5V (e.g., +4.9V) than when I use above 5V (e.g., +5.1V).

Our circuit is a typical differential amplifier using two opamps included in the same package and some high precision resisters.

The measuring parameter is zero drift characteristics, and the input is biased about +2.5V by the same value resistors, and two amp's +input is shorted.

Temperature condition is +25℃ (const.) using a chamber.

I think different parameter is just the supply voltage.

Actually, under investigation, output is very clear and almost no drift when using +5.1V power supply.


Do you know the reason of this problem or the true recommended operating conditions?

I thought I should use over +5V power supply on OPAx227 series.

Best regards,

  • 0
    TI__Mastermind 23610 points

    Hello, 

    The power supply can be 4V to 36 V on a single supply. See below.

    The input common mode voltage range is 2V inside each supply rail for linear operation. The common mode range for linear operation is circled below and these are the min and max points. Going outside of these boundaries can lead to unexpected results. I would double check that the common mode voltage is set to 2V below the positive rail. Because the linear operation of the device is limited to the common mode range shown below I would recommend operating at the higher supply voltage that is working for you. It will just make it easier to achieve the linear operation point of the device. 

    I hope this information is helpful. Let me know if I can be of further assistance. 

    Best Regards, 

    Chris Featherstone

  • 0 in reply to Chris Featherstone
    Prodigy 20 points

    Hello, Mr. Chris.

    In this case, the single supply voltage is greater than 4V so it doesn't be a problem.
    Also, the recommended input common mode voltage range is 2V to 3V when using Vs=5V.

    Section 6.7 Electrical Characteristics is measured by Vs=±5V to ±15V so I guess those fundamental performance are not shown.

    Best Regards,

    Kazuyuki Miyashita

  • 0 in reply to Kazuyuki Miyashita
    TI__Mastermind 22447 points

    Kazuyuki,

    Chris is on vacation, so I will help for the time being.  

    1. Most important:  you mention OPA2277 and OPA2227.  These are two different devices.  The OPA2277 has a minimum supply of 4V and the OPA2227 have a minimum supply of 4.5V.  Please clarify which of the two devices you are using.
    2. I think the likely problem is common mode range.  Chris mentioned this but I would like a little more detail on your circuit to confirm we are not violating this specification.  
    3. To help me understand the common mode situation for your circuit, can you please post a schematic of your circuit with measured input signal, output signal, and supply voltages.
      1. Note that the minimum operating supply range for OPA2277 is 4V (see table 6.2).
      2. Also, note that the common mode range is 2V above the negative supply and 2V below the positive supply.  If your supplies are 0V and 4V this means that the circuit will only work for Vcm = 2.000V.  If your common mode is off a little you will get non-linear results.  I would not recommend operating with such a small window of safety margin.  You could use a device like OPA186 which   
      3. Op Amp Input and Output Swing Limitations covers an explanation of common mode ranges.  If you are not familiar with common mode range and the methods for calculating common mode ranges please read this document.
      4. Most op amps that have wide voltage supplies (e.g. 36V) have limited operation at low supply (e.g. 4V or 4.5V).  If you really need an op amp that can operate at 4V supply up to 36V supplies you might look at OPA188.  This device has a wider common mode range than OPA277.  The device also is optimized for good offset similar to OPA277. 
      5. Sometimes it is necessary to use a 36V op amp for low supply applications (4V).  However, if you really only need lower supply voltages you may want to switch to a low voltage op amp.

    Best regards, Art

  • 0 in reply to Art Kay
    Prodigy 20 points

    Dear Mr. Art,
    First of all, I regret my mystakes of part number. I was going to discussing about OPAx227 series.
    I changed the part number of this forum.

    1. I'm using OPA2227.

    2. The supply voltage is +5V single supply generated by a regulated power supply.

    3. Here is our circuit.

       Unfortunately, due to including technical the information, I'm not able to provide all of it here.  

       The ADC data rate is 1 sps and using  a 0.1 Hz digital lowpass filter.
    　

    There two inputs are connected to Vs / 2 so these outputs should have few drift when environment is kept to constant temperature.

    This is sampled data when Vs=5.1V. The vertical axis of the graph is normalized to 5V.

    I think data is very stable.

    However, the measured data by Vs=4.9V seems a bit unstable and noisy.

    I tested this experiment with the same 10 circuits under the same conditions, and they behaved all similarly.

    In fact, the problem is still under investigation and may be occurring in our circuit or enviroment.

    I'm going to investigate to isolate the problem later (e.g., measuring the indipendent drift of out equipment).

    Best regards,

    Kazuyuki Miyashita

  • 0 in reply to Kazuyuki Miyashita
    TI__Mastermind 22447 points

    Kazuyuki,

    1. In the last communication, you mentioned that the problem happened at 4V.  In this post you say that the problem happens at 4.9V.  4V is out of the range for recommended operating conditions.  4.9V is inside the limit.  This inconsistency is very important.  Please double check your circuit and confirm conditions.  See table 6.3 for supply range.
    2. The output shown in the two plots above shows a voltage of about -0.018V.  I assume this is the differential output of the amplifier, so I don't really know the output of each amplifier.  Based on the schematic, the output of each amplifier should be approximately 2.5V.  The input should be inside the common mode range of opax227 as the common mode range should be +2.0V to +3.0V and the input signal should be at 2.5V.  Please confirm outputs.
    3. Assuming the supply is 4.5V to 5V the circuit should work as drawn.  However, I suspect based on the previous discussions that this is not the case and the problem is supply voltage and/or common mode range.  If you are keeping the supply in the range of 5V (e.g. 4V to 5V), I would suggest using a low voltage op amp like OPA325.
    4. Since your readings are being done with an ADC you may be experiencing ADC limitations and/or an interaction between the ADC and op amp circuit.  I suggest repeating the experiment with the ADC disconnected and the output being monitored with a precision voltage meter(34401a or better).

    best regards, Art

  • 0 in reply to Art Kay
    Prodigy 20 points

    Dear Art,

    Thank you for your opinion.

    I'll do more research later and share if I found issues.

    Best regards,

    Kazuyuki Miyashita