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LMH5401: Output

Cedrick
Expert 8270 points
Part Number: LMH5401
Other Parts Discussed in Thread: TIDA-01350, , OPA855

Hi,

Good day.

Our customer would like to use the in their project and they have question described below. I hope you could help us with this.

"ok so my task is:
I have a Photodiode wich gives me an Output Voltage of around 100nA and the Signal is oscillating with a frequency of 8Ghz. Like very simple: a sinus with an amplitude of 100nA and frequency of 8 Ghz. I wanted to know what is the Gain or like the actual Output after the TIA and what Noise is added onto the signal, so the Signal-to-Noise ratio. I hope this makes it a litlle more clear. Since i am not so sure how to calculate the actual output and SNR of the system.

i meant photocurrent not Voltage of the photodiode"

Thank you, Looking forward to your support.


Regards,

Cedrick

  • 0
    Guru 140200 points

    Hi Cedrick,

    you mean like this?

    What is the detector capacitance of photodiode?

    Kai

  • 0 in reply to kai klaas69
    TI__Expert 8270 points

    Hi Kai,

    Thank you for your response

    The customer informed me that the detector capacitance is 220fF, max. 240fF.


    Regards,

    Cedrick

  • 0 in reply to Cedrick
    TI__Mastermind 24209 points

    Hi Cedrick,

    Your detector capacitance is lower than that of the detector used in TIDA-01350.  Within this reference design, the transimpedance can range from 500-10000Ohms.  If you refer to section 5.3 of this reference design, there is a discussion about transimpedance and diode capacitance affecting bandwidth.  Smaller diode capacitances and/or smaller TIA gains will enable higher frequency operation.  The current reference design can approach 1GHz.  You may be able to realize higher bandwidth due to your choice of detector (and necessary TIA gain).

    Are you curious about how the LMH5401 can handle an 8GHz sine wave a 100nA (functioning as a TIA)?  Or are you curious how the LMH5401 can perform with sinusoidal input at 8GHz?

    If you could elaborate on your customer's TIA gain needs and any other specs they are concerned with, I can help you find the right part.

    Best,

    Alec

    High Speed Amplifiers

  • 0 in reply to Alec Saebeler
    TI__Expert 8270 points

    Hi Alec,

    Good day. I've just got a feedback from our customer. Kindly refer below.

    "i will try to elaborate it a bit more.
    So i have a transient sinusidal ( light source) signal that oscillates with a frequency of up to 8 Ghz. ( i can tune it done to 6 Ghz min)
    With my current Photodiode ( 10 ghz bandwidth) i can detect the signal. But the problem is, that the signal is very very weak. Aprox. 100nA Photocurrent is generated in the photodiode( dark current of the pd is 6 nA). And i want to digitalize the Signal and look at it on an Oscilloscope or look at it "live". But with that low Photocurrent it is impossible to show it on an oscillocope ( at least fo me).
    Therefore i would like to amplify the Photocurrent of the Signal with the TIA to at least mV regime for the oscilloscope.

    So to speak is it important to know wich gain does a 100nA Photocurrent Signal with a Frequency of 8 Ghz gets from the TIA.And if you have a additional Idea how to solve the Problem ( maybe with a lock In amplifier or something else) i am open ears"


    Looking forward to your inputs.


    Regards,

    Cedrick

  • 0 in reply to Cedrick
    TI__Mastermind 24209 points

    Hello Cedrick,

    Thank you for the elaboration.  My team and I will look into this.  Your 'gain' will be based on the TIA's transimpedance; in TIDA-01350 the default range is from 500Ohm to 10000Ohm.  For a 100nA photocurrent and 10000Ohm transimpedance (the TIDA-01350 default); the resulting voltage could be (-)5mV depending on your photodiode / current orientation AND if your output voltage swing is from 0V to 5mV.  A larger TIA gain than 10000Ohm might be desirable for your use case.

    There is a short TIA guide which may help: https://www.ti.com/lit/an/sboa268a/sboa268a.pdf?ts=1615906336335&ref_url=https%253A%252F%252Fwww.google.com%252F

    A typical approach when designing TIA circuitry is to know your input current (range) and output voltage range; you can select your transimpedance from these two values (using Ohm's Law).  This is covered on page 2 of the guide.

    Best,

    Alec

  • 0 in reply to Alec Saebeler
    TI__Expert 8270 points

    Hello Alec,

    Good day. Sorry for the delay on my response.

    I've just got a feedback from our customer. Kindly see their response below.

    thanks for your help.
    But i seems that a TIA can not achieve the gain factor i am looking for at these frequencies.


    Do we have other device that we could offer to them in this case? Thank you.


    Regards,


    Cedrick

  • 0 in reply to Cedrick
    Guru 140200 points

    And is the customer giving any details why this seems to be so?

    Kai

  • 0 in reply to Cedrick
    TI__Mastermind 24209 points

    Hello Cedrick,

    Please see the two threads below; they might provide some context for your design.

    https://e2e.ti.com/support/amplifiers/f/amplifiers-forum/734622/lmh5401-lmh5401-for-transimpedance-amplifier?tisearch=e2e-sitesearch&keymatch=LMH5401#

    https://e2e.ti.com/support/amplifiers/f/amplifiers-forum/987143/opa858-another-photodiode-amplifier-for-short-1-2ns-laser-pulse

    I would suggest for your application you could use a pairing of the OPA855 with the LMH5401; you would use the OPA855 as a transimpedance amplifier & use the LMH5401 to drive the ADC or further amplify signal.  This is an option you could pursue.  

    Best,

    Alec