About the opa445

Discussion in 'General Electronics Chat' started by Truman, Aug 31, 2011.

  1. Truman

    Thread Starter New Member

    Aug 30, 2011
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    Dear all,

    My name is Truman.

    I have two questions about the OPA445.



    First, about the "package-lead", there are 4 kinds of the package,

    "DIP-8", "SO-8 Surface-Mount", "SO-8 PowerPAD" and "TO-99 8-Pin".

    What's the difference in the performance between these 4 kinds of the package?



    The second question is, recently, I bought a OPA445AP to design a power amplifier.

    The power amplifier is chose to be inverting power amplifier.

    The input resistor and output resistor are 3M Ohm and 12M Ohm, so the gain is 12M/3M=4 if neglecting the minus sign.

    According to the datasheet of OPA445, the supply voltages of OPA445 could as high as +/- 45V.

    However, when applying +/- 40V, in addition to the output signal is disappeared from the oscillator scope,

    the temperature of the OPA445AP is quite high.

    But, when applying +/-30V, the performance is quite well and the temperature is moderate.

    So, what's the problem in the OPA445 or my circuit ?

    Is anyone can help me?

    Thank you very much.









    Truman
     
  2. SgtWookie

    Expert

    Jul 17, 2007
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    The TO-99 is the only hermetically sealed package offered. It's also the worst thermal performer at 200°C/Watt. The best thermal performance is the SO-8 PowerPAD at 52°/Watt. The DIP-8 is the next-best thermal performer at 100°C/Watt, and is convenient to breadboard - as long as one is mindful of the power dissipation limits when used on a breadboard.
    Datasheet: http://www.ti.com/lit/ds/symlink/opa445.pdf
    That is the DIP-8 package version.
    Your resistors are quite high in value. That will cause the circuit to be vulnerable to electrical noise.

    You need to supply a schematic of your entire circuit.
    You either have a rather large load for the output of the opamp, or you have a reactive load that is too great for the frequency you are attempting to drive with the opamp.
     
  3. Truman

    Thread Starter New Member

    Aug 30, 2011
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    Thank your generous replying.

    Quote:
    The power amplifier is chose to be inverting power amplifier.
    The input resistor and output resistor are 3M Ohm and 12M Ohm, so the gain is 12M/3M=4 if neglecting the minus sign.
    Your resistors are quite high in value. That will cause the circuit to be vulnerable to electrical noise.
    ==>The reason why I use these high value resistors is that since the temperature of OPA445AP is quite high when applying +-40V, use high value resistor may helpful to reduce the output current. And then, the temperature may become cooler than use low value resistor, such as 2k Ohm and 8k Ohm. But, obviously, it does not work.

    http://www.badongo.com/pic/13654652
    I put the image of my circuit on "bandongo.com".
    Well, I build my circuit on breadboard, and monitor the input/output signal by an oscilloscope.
    Because I build this circuit in order to drive a PZT to scan a laser, in other words, the frequency must be low value. Actually, the input signal is a sine wave with frequency about few hundred Hz when testing the circuit. But it is useless.

    Please help me. Thank you very much.
     
  4. SgtWookie

    Expert

    Jul 17, 2007
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    Going back to the datasheet, you will see under "Power Supply" on page 3, that quiescent current is typically 4.2mA when Vsupply=±40v, but can be as high as 4.7mA when Vsupply=±45v.

    You're trying to use ±40v, so 4.2mA * 40 * 2 = 336mW power dissipation without the opamp doing anything but sit there.

    As I indicated before, thermal rise is 100°C/Watt for the DIP package; so .336*100=33.6°C temp rise over ambient. Max operating temp is 125°C. If you started off at room temp (25°C), then you would be @ 58.6°C. The 12MEG load adds 133uW to the power dissipation; not much.

    So, I have reason to suspect that you have something else connected to your opamp output, or you are trying to operate the circuit inside a box with no air flow, or you are exceeding the supply voltage limits, or the common mode input limits.
     
  5. Audioguru

    New Member

    Dec 20, 2007
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    The opamp might be oscillating at a high frequency (causing heating) due to the high stray capacitance of the breadboard. Use a compact stripboard or compact pcb layout instead.

    The leads on the IC conduct away heat to the copper wiring of a stripboard or pcb that does not happen when you plug the IC into the contacts of a breadboard.
     
  6. Truman

    Thread Starter New Member

    Aug 30, 2011
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    0
    SgtWookie:
    The output signal first goes to the upside of the oscilloscope and then disappears. Later, it goes back to the screen, but with the strange shape. Also, the gain isn't right.
    I think there is nothing else connected to my opamp output except an oscilloscope. And I build the circuit on a breadboard, so I guess I can assume the air is flowing. But, I don't know what is common mode input limits. Could you explain it for me? Thank you.

    Audioguru:
    Ok, I will try to build the circuit on stripboard.
    Thank your suggestion.
     
  7. Audioguru

    New Member

    Dec 20, 2007
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    The allowed input common-mode voltage range is the range of input voltages at a certain supply voltage where the inputs work normally. The datasheet for the OPA445 spec's an input common-mode voltage range that is 5V from either supply voltage. So with your plus and minus 40V supply, the allowed input voltage range is from -35V to +35V.

    Some other Fet-input opamps have an input problem called "phase inversion". The output suddenly goes as high as it can when an input goes outside its allowed common-mode voltage range. Maybe the OPA445 also has this problem and maybe your input goes higher or lower than plus or minus 35V. The TL07x and TL08x have this problem and is shown on my 'scope shot:
     
  8. SgtWookie

    Expert

    Jul 17, 2007
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    Well, if the OP's supplies are ±40v and the input signal doesn't exceed ~±8.5v, they should not have any clipping or phase inversion. I downloaded the models for the OPA445 and OPA445E from TI's website; they don't seem to exhibit phase inversion, but such models can leave a few things to be desired... :rolleyes:
     
  9. Truman

    Thread Starter New Member

    Aug 30, 2011
    4
    0
    Thank your generous help.

    Well, I wire an e-mail to TI.
    According to the letter they reply to me, the problem seems to be the package.

    Here is some part of the letter:
    2)+-45V power supply has no problem to OPA445, and with the higher power supply, the more power dissipation increased, correspondingly, the output current also increased to caused the temperature raised high,so special attention should be paid that ,soldering the exposed thermal pad to a copper (as big as possible) plane on the PCB provides good thermal performance. In most cases the largest copper plane on the PCB will be GND plane. Please confirm it again.

    I think maybe the problem is the package type. The OPA445 I bought is plastic type without exposed thermal pad, but the TI's engineer assumes my OPA445 is PowerPAD type. I know the difference between plastic type and PowerPAD type is the thermal resistance. But if the thermal resistance is a huge problem, the datasheet should denote this in the electrical characteristic. However, it is not the case.

    Well, I have write an e-mail to TI with this question, but up to now, they have not reply any thing to me. I think the most worse case is buy a new PowerPAD type of OPA445, but I wish I don't need to buy in order to save some money.....
     
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