LMH6639: Initial operation at power-on

Hello Daisuke Matsuoka,

   Thank you for confirmation via debugging. Quick question do you have an input applied at the amplifier before supplying 3.3V to the amplifier? If so, what is the input range? 

Best Regards,

Sima 

Hello Sima Jalaleddine,

Thank you for your prompt response.

The answer to your question is “I don't know.

The reason is that there is an op-amp in the stage before the op-amp in question, and there is also a voltage divider resistor in the stage before that.

It is difficult to explain in text alone, so I am attaching a schematic diagram of the relevant part.
Please check it.

The voltage divider, the op-amp in the first stage (TLV8541), and the 3.3V used for the op-amp in question (LMH6639) are all from the same power supply.

Please contact us if your answer lacks any information.

Best regards,

Hello Daisuke Matsuoka,

  The schematic was helpful! I do not see an issue since they are all using the same power supply. The other question would be if REF pin is supplied prior to the 3.3V supply?

   I haven't run into this issue before, but will ask my team by tomorrow if they have seen this occur. But, if the answer is yes for above question, I am thinking it could be that if there is an input applied and amplifier before startup, then when supplies are applied takes time to go into disable mode which is typically 160nsec from datasheet. Is that around the time duration you are seeing the output, and if possible can you probe at the input of the amplifier when the 3.3V supply is off? 

Thank you,

Sima 

Hello Sima Jalaleddine,

In response to your question, I have experimented with probing the circuit and am attaching the results. The measurements were taken at different times: 50ms, 5ms, and 1ms.

In all figures, Channel 1 (yellow) shows the 3.3V at pin 7 of U3 in the schematic, Channel 2 (green) shows the REF at pin 3 of U3, and Channel 3 (blue) shows the output at pin 6 of U3.

Regarding the circuit, I noted some mistakes in my previous version, which I have now corrected. My apologies for any confusion caused. Specifically, I have changed the resistor and capacitor values in the output circuit of the LMH6639 and the resistor value in the voltage divider.

Concerning another of your questions, I have experimentally confirmed that the REF pin is not supplied prior to the 3.3V supply.

You mentioned that the issue might be linked to the op-amp entering disable mode approximately 160ns after power is supplied. However, the relevance of this to the experimental results is not clear to me.

Can you explain, based on the experimental results, whether the op-amp outputs momentarily only at the moment of power-on, even if the SD signal is at 0V?

Best Regards,

Hello Daisuke Matsuoka,

  Thank you for these tests and the schematic update. Your observation is correct that the output getting to 0V is much longer than the shutdown stated in earlier reply. And since your tests show the REF pin is on around the same time as the 3.3V supply, the theory from my earlier reply would not be relevant to what is occurring for your situation. It looks like a voltage that is discharging from some sort of capacitance.  

   Looking at the schematic, there really shouldn't be any voltage feeding back to the input of the amplifier with this type of configuration due to the N-Channel Enhancement type MOSFET: 

    But as debugging steps, would you be able to either disable the bias voltage to the FET, or remove the 200Ohm resistor, or do both separately? 

    After doing the above steps, and if you see no change, then please remove the 1500pF (1.5nF) capacitance, and then also observe the output. 

Thank you in advance for trying out the steps above,
Sima