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OPA188: Instability of OPA188

Ernest Moorey
Prodigy 70 points
Part Number: OPA188
Other Parts Discussed in Thread: OPA187,

We have been using the OPA188 in a number of new development and pre-production projects. We have obtained the OPA188 from three different sources and they all have the same major problem. They are unstable and very noisy. When replaced with the OPA187, the problem is not present. When replaced with any other amplifier, the problem is not present. Does TI have a problem with the OPA188? This device has a very high specification and we would very much like to use it, but we have had to remove this from all products until the problem is solved. Please let me know that there is hope of this very high specification device. All the devices we have obtained from three different sources have the same problem. With kind regards, Ernest (Moorey)

  • 0
    Guru 47495 points
    No way to answer this without a circuit - ideally a TINA file,
  • 0
    Guru 140200 points
    Hi Ernest,

    no, the OPA188 has no special stability issue and as Michael already mentioned a schematic would be helpful. Also, please specify what you mean by "instability". A scope plot would be helpful.

    That said, some people have problems with the OPA188 and other auto-zero OPAmps because of the internal chopper switchings. See section 8.3.3 of datasheet of OPA188. One remedy is to decrease the resistance in series to the +input of OPA188. Keep it as low as possible and/or mount a filtering cap from the +input of OPA188 to signal ground. The chopper switching noise coming out of the +input pin of OPA188 should see a low ohmic path to signal ground.

    Kai
  • 0 in reply to kai klaas69
    Prodigy 70 points

    Dear Kai and Michael,

    Thank you for your reply.

    I have attached a schematic of one of the circuits that we are using the OPA188 in.

    IC5 was also a OPA188 but we changed that for a OPA187, to try and cure the problems.

    I will try and get a computer readout, but this will take a lot of time as we are not fully working with the scope.

    It may have to be a .jpg.

    We have tried this device in a lot of different circuits and the same problems occur in all devices.

    As I stated earlier, the problem is always fixed if I use any other amplifier in place of the OPA188.

    We really want to use this device as the spec is just about ideal for us.

    With kind regards,

    Ernest

     

    I tried to reply to Kai’ reply, but it was not allowed.

    I hope this one gets through.

    I have tried to insert the .pdf of the schematic of one circuit.

     

  • 0 in reply to Ernest Moorey
    Guru 47495 points
    I don't see a schematic, use the Insert code to the lower right, which will open a new window then hit the paperclip symbol to insert file.
  • 0 in reply to Michael Steffes
    Prodigy 70 points

    Dear Michael,

    I have inserted the schematic, a .pdf, but I did that before. Anyway i hope you get this one.

    OI should point out that I have had the problem on all the designs using the OPA188.

    With kind regards,

    Ernest

  • 0 in reply to Ernest Moorey
    Guru 140200 points

    Hi Ernest,

    hit the button "Insert Code, Attach Files and more...". Then hit the paperclip symbol, as Michael already mentioned:

    Kai

  • 0 in reply to kai klaas69
    Guru 47495 points
    yea, we are not seeing anything attached to look at - as you start to reply, look at the lower right and click that "Insert Code, Attach Files and more..."
  • 0 in reply to Michael Steffes
    Prodigy 70 points

    I71b Magnetic Isolator Sch.pdfDear Michael,

    Here is another attempt.

    I hope this one is OK.

    With kind regards,

    Ernest

  • 0 in reply to Ernest Moorey
    Guru 47495 points
    Whew, that is a bit much to take in - that upper right area looks like a Sallen Key filter maybe? Do you have a frequency of oscillation?
  • 0
    TI__Guru* 93105 points
    Hi Ernest,

    We definitely need to see your OPA188 circuit schematic as Kai and Michael have indicated. Please include the input signal, power supply and output load information. The more information you can provide us the better.

    You mention "We have obtained the OPA188 from three different sources and they all have the same major problem. They are unstable and very noisy." and "Does TI have a problem with the OPA188?" The OPA188 is designed into many applications where it meets its electrical specifications and performs as expected. Were the 3 different sources from whom you obtained the OPA188 product a distributor other than TI directly, or an authorized TI distributor?

    Regards, Thomas
    Precision Amplifiers Applications Engineering
  • 0 in reply to Thomas Kuehl
    TI__Guru* 93105 points
    Hi Ernest,

    We can now see your schematic - thanks.

    I am seeing a number of 100 k, I Meg and even 10 Megohm resistors being used in the active filter circuits. The OPA188 uses a chopper architecture. There is charge transfer associated with the switches in the chopper circuits. The charge transfer associated with the switching produces current transients that modulate the nominal input bias current. The effect is not usually observed with low and moderate value resistors through which the input current flows. However, it can become more evident when that input current flows through very high resistances. The low-level current noise is converted to voltage noise that is observed at the op amp output.

    The OPA188 has wider bandwidth and a higher internal chopping frequency than the OPA187. Therefore, the effect may be more pronounced in your application - if indeed that is the source of the observed noise.

    Regards, Thomas
    Precision Amplifiers Applications Engineering
  • 0 in reply to Thomas Kuehl
    Prodigy 70 points
    Dear Thomas,
    Thank you for your reply.
    I can understand the reasoning behind the choppers giving charge to the input.
    This makes sense, although I am surprised that it makes such a lot of interference to high impedance circuits.
    We can transfer to the OPA187, which does seem to work in all the circuits we are developing at the moment.
    My query has been addressed by two of your colleagues as well, Michael and Kai.
    You will know the response you have had to the OPA188 and you will know how they have been received.
    If other users have not had the problems we faced, then your explanation does fit the symptoms.
    Both of your colleagues have said that the part does not seem to cause any widespread problems, so I think that we will transfer to the OPA187.
    The OPA188 does have a better noise performance than the OPA187, but the latter uses less current, so a compromise must be reached.
    Anyway, I am impressed with the performance of both these parts.
    I am now going to try and send this email to you by the website, as it is a no reply one and my previous attempts were unsuccessful.
    If you can, would you please pass these comments on to Michael and Kai.
    With kind regards,
    Ernest
  • 0 in reply to Ernest Moorey
    Guru 140200 points

    Hi Ernest,

    auto-zero OPAmps using a chopping scheme to provide an ultra low input offset voltage like the OPA188 are intended to be used with low ohmic sources like bridges, strain gauges for electronic scales, etc. What all of these applications have in common is that a very low signal has to amplified by a very high gain (>100) and that all the feedback resistors are very low ohmic to keep the noise low. This is the typical application where the OPA188 can demonstrate its excellent performance!

    I think it was Marek who carried out this remarkable noise analysis:

    6747.2188 Input Bias Current Commutation.ppt

    As you can see on page 11 of this paper, at high gains and with low ohmic feedback resistors (100R and 10k, e.g.) the chopper noise is not a dominant noise contributor.

    Kai

  • 0 in reply to kai klaas69
    Prodigy 70 points
    Dear Kai,
    Thank you for your reply.
    I have been involved in the design and development of ultra low level DC amplifiers since 1965 (yes really!)
    That was when I was with Dynamco.
    If you look at our website, you will see some of the applications. EM website is at www.emelectronics.co.uk
    Some years ago I carried out consultancy work for Keithley Instruments and I used to know dear old Joe F Keithley as well as his son JP.
    EM Electronics has been in business since 1979.
    I appreciate the point about charge injection at the input and we are getting charge injection to lower than 1pC pulses.
    We are finding that the OPA187 is working in our circuits like the isolator circuit I sent you and that is the part we will be using in future.
    We would have liked the 0.1Hz to 10Hz noise figure of the 188 at 250nV pk/pk, but we will settle for the 187 at 400nV pk/pk.
    I would like to thank you, Thomas and Michael for the time you have taken and for the help you have given in assisting our decision for this part.
    With kind regards,
    Ernest