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fearing damage of OPA699 with saturation

Shyam
Expert 1960 points
Other Parts Discussed in Thread: OPA699

one of my design involves pulse signal processing from photodiode, i am using OPA699 as a limiting amplifier stage before to comparator and next to preamplifier, current gain i have set for OPA699 is 10 using 680 and 68 resistors


i want to know what happens when OPA699 saturates


op-amp stage just before OPA699 is LTC6269-10 which on saturation gives a output current of 10mA and full swing of -2.5V , what i observed on simulation is, on saturation of OPA699 there is voltage of 1.2V built at the inverting terminal, ideally it must be very less, there is a current which is entering in to the inverting terminal of about 2mA , why is this happening?

when i see the data sheet it clearly says the maximum bias current is 10-20uA

am i trying to damage the device ?

so what i want to know is when OPA699 is given a input of 2.5V @ 10mA current given output giving 4.3V and opamp being saturated, why do i see a current entering exceeding limit the uA is entering in to inverting input, will this damage the opamp ???


answer me quick please, i am done with design and this is blocking


i also observed

  • 0
    TI__Genius 12631 points
    Please provide a circuit schematic or the TINA file you are using. Have you observed this phenomenon on the bench or just in simulation?
  • 0 in reply to Samir Cherian
    Expert 1960 points

    I have the circuit done in LTspice because of non-portability of LT chips, I took the spice model from TI and imported it to LTspice, I hope it is not the problem, please tell me the behavior of OPA699 in saturation, anyhow I will also try in TINA 

    the circuit looks like below

    schematic

    i have removed the 100f capacitor in the feedback, and for simulating D3 i am using a pulsating current sources

  • 0 in reply to Shyam
    TI__Genius 12631 points

    I looked at the circuit in TINA and it works fine. I used +/-5V supplies and maintained the clamps at +/-4V. I put a 1MHz 6Vpp output signal and it looks good. The bandwidth of the part is 118MHz in a closed loop non-inverting gain of 11V/V. You can see the saturated response of the amplifier on page 8 of the datasheet.

    In your simulation I do not see what power supplies and clamp supplies are being used. Also, can the LTC device drive a 68ohm resistor load and supply the necessary output swing?

    -Samir

    OPA699.TSC

  • 0 in reply to Samir Cherian
    Expert 1960 points

    i tried to simulate the first stage in TINA and has attached the same, you can see the input current going in 100's of uA, remaining two opamp datasheet specifies me what is the maximum current limit for their input terminals, but i dont see any such in case of OPA699 so i am worried about the damage of opamp

    0676.OPA699.TSC

    please note that the op-amp is connected in inverting mode, when the LTC-6269 saturates i have seen it giving a current of 10mA, below is the design

    the U4 is a protection diode to bypass excess current, as the LTC6268 requires inverting terminal input voltage should not exceed 2V, so the protection diode, i have given a input of 200mA up on which the first stage saturates and gives out current of 10mA that can be observed across R8(shown in figs), now when i see the current into the inverting terminal of opa699 it exceeds the limit of 10uA(Ix(U3) in plot) so i am worried 

    if you see LTC data-sheet it clearly says what is the maximum current that input terminal can bear that is 10mA, in the same way what is the maximum current that can enter in to inverting terminal 30mA? page 23 speaks like that

  • 0 in reply to Shyam
    TI__Genius 12631 points
    The OPA699 should be able to handle around 5-10mA of current but this is a fault current. There is no reason for a linear current in the mA range to flow into the amplifier. Desolder the R8 (68 ohm) resistor and see if the output of the amplifier stays close to GND. If it is railed or constantly moving then the amplifier has probably sustained some long term damage. The 10mA limit is fairly typical and is based on the size of the input protection diodes.

    So lets assume that the output of the LTC device goes all the way to 2V; since the amplifier is inverting the current through the resistor R8 would be 2/68 = 30 mA. However 30mA*680 is 20V which the amplifier cannot go to so the output would saturate at the clamp limit. This means that the excess current will probably start flowing into the ESD protection diodes which would then get damaged and damage the whole device. I suspect that the output clamp is not protecting the amplifier inputs here. In a traditional opamp when the output clamps the amplifier will stop operating in the linear region and the inputs would begin to separate which would work to limit the current into the ESD diodes. However I suspect that is not happening here. Can you try switching into a non-inverting configuration and see if this fixes your problem. I will have to speak with some of the designers to see what kind of input protection is present on these devices.
  • 0 in reply to Samir Cherian
    Expert 1960 points

    do i need to use a protection diode for the OPA699 stage also, to bypass the excess current, as i was doing at the 6269  ?

  • 0 in reply to Shyam
    TI__Genius 12631 points
    Hello Daram,
    A protection diode will help. Protection diodes tend to add a capacitive load so that may needed to be compensated by adding a feedback cap in parallel with the 680ohm resistor.
  • 0 in reply to Samir Cherian
    Expert 1960 points

    in fact the inverting terminal is showing -1.8V after saturation(this usually happens with opamps, as they build up voltage after saturation) so if the input stage saturates input is -2.4, voltage difference divided by 68 always shows me 10mA , as opa699 is saturated the current through 680 will be 4.3+1.8(voltage built at inverting terminal)=6.1/680=8.9mA remaining 1mA is observed to flow in to inverting terminal i think

    i feel this is safe and i can go with this, because my inverting terminal is not exceeding the voltage limit and neither the current in to terminal is not exceeding 10mA

  • 0 in reply to Shyam
    TI__Genius 12631 points

    What is the output of the LTC when you observed this....is that the -2.4V?

  • 0 in reply to Samir Cherian
    Expert 1960 points

    yes its Vcc is +/-2.5V,so its limited by power,  do you think any problems with my design approach ? 

  • 0 in reply to Shyam
    TI__Genius 12631 points
    Daram,
    I think you are okay. I spoke to the designer and he mentioned that the internal design has a secondary stage which will absorb the extra current when the output is clamped.
    So what voltages are you clamping the output of the OPA699 at (4.3V)? Also, what happens to the intermediate voltages and currents if you lower the clamps to say 3V?
    -Samir
  • 0 in reply to Samir Cherian
    Expert 1960 points
    i am not clamping intentionally, its the vcc limit i think so, the opa699 output is given stratight to comparator for further processing, you can have a relook at the schematic for finding any flaws further, the only problem with opa699 is TI is not willing to provide any samples, why so i did not understand the intention, is this chip going to be discontinued ?
  • 0 in reply to Shyam
    TI__Genius 12631 points

    Daram, The OPA699 is not in danger of being discontinued. There may be a bug in the system. I can see that it cannot be sampled if you go from the main TI website, however if you go directly to www.estore.ti.com you can request samples there.

    -Samir

  • 0 in reply to Samir Cherian
    Expert 1960 points
    i had done from estore also, the same problem is re occuring please help in getting samples of OPA699 , both of my LT samples are with me for I-V conversion and A/D but the crucial OPA699 for Limiting and inversion is not there, please make it urgent in providing me samples as soon as possible
  • 0 in reply to Shyam
    TI__Genius 12631 points
    Daram,
    I just placed an order online and it went through with no issues. What problem are you seeing? Where are you located?

    -Samir
  • 0 in reply to Samir Cherian
    Expert 1960 points

    with is problem i am not able to checkout

  • 0 in reply to Shyam
    TI__Genius 12631 points
    Daram,
    Thanks for the screenshot. I have forwarded this to the samples team to see if they can find the root cause of the issue. I will let you know once I have heard back.
    -Samir
  • 0 in reply to Samir Cherian
    TI__Genius 12631 points
    Daram,
    I just got a response from the samples team; TI policy does not allow shipping free samples to users with a non-business or non-university authorized email address. This could be the reason for the error you are seeing.
    -Samir
  • 0 in reply to Samir Cherian
    Expert 1960 points
    ok i will try with my office mail id with new registration
  • 0 in reply to Shyam
    TI__Genius 12631 points
    Daram,
    If your order works, can you please close this thread. You have a few open threads on this forum. It is important that we close out threads otherwise it messes up our internal metrics.

    -Samir