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LM98714: LM98714: Target Black Level Code Selection

Noniel Paul Novabos
Prodigy 200 points
Part Number: LM98714
Other Parts Discussed in Thread: LM98620

Hi,

I'm new to CCD applications and I'm planning on using the LM98714 as Clock generator and AFE for  TCD2905. I was hoping to ask for tips in choosing the Target Black Level code.
My CCD output's black level is somewhere around 6V and saturated level is around 3.5V. How do I choose the Target Black Level code? Should I choose a Full Scale code? or should I have a guardband for that, and if so how much guardband?

Appreciate your support on this.

Many Thanks,
Paul

  • 0
    TI__Mastermind 24855 points

    Hi,

    Thank you for your interest in LM98714. Let me review your question and feedback to you by Feb 16th.

    regards

    Shinya 

  • 0 in reply to Shinya Morita
    TI__Mastermind 24855 points

    Hi,

    LM98714 cannot accept 6V/3.5V input directly from CCD. Please refer page 18 as typical AFE/CCD interface.

     

    Thanks,

    Regards

    Shinya

  • 0 in reply to Shinya Morita
    Prodigy 200 points

    Hi,

    Yes, I'm planning to use the Input clamping and the PGA to fit this input signal into the range specified in the ADC, but my question is how do I choose the Target Black Level code in Page 0 Registers 10 - 11? I believe this is the target ADC output during the Auto Black Clamp Loop.

    Thanks,

    Paul

  • 0 in reply to Noniel Paul Novabos
    TI__Mastermind 24855 points

    Hi,

    Thank you for your question. Let me review your question and feedback to you by Feb 19th.

    regards

    Shinya 

  • 0 in reply to Shinya Morita
    TI__Mastermind 24855 points

    Hi,

    As you expected, target black level can be set with Page 0 register 10-11. This is ADC output code. Black level of ADC output will be affected by Black level offset DAC before ADC(Fig 22 datasheet page 28). Offset DAC will be adjust to meet target black level(ADC code) during auto black level correction loop.Unfortunately we do not have register example for LM98714(LM98620 has example code), need to set registers based on customer want.

    regards

    Shinya

  • 0 in reply to Shinya Morita
    Prodigy 200 points

    Hi,

    I see. So for my CCD sensor, as I mentioned previously, my expected dark level is at 6V and saturated level is at 3.5V (dark level is higher than saturated level), then I'll use the PGA to attenuate the signal to fit within the ADC input range. So am I correct to set the target black level to a value close to full scale?

    Thanks,
    Paul

  • 0 in reply to Noniel Paul Novabos
    TI__Mastermind 24855 points

    Hi,

    LM98620 analog input range is up to 1.2V peak to peak only.(Datasheet page 7). If you plan to input 2.5Vpp(6V-3.5V), it beyond the acceptable range of LM98620 input.

    regards

    Shinya

  • 0 in reply to Shinya Morita
    Prodigy 200 points

    Please note that I am using LM98714, not LM98620. I never mentioned using the latter. LM98714 has an input range of 2.3V when CDS gain = 1x. And since my maximum range is still beyond that by 0.2V, I mentioned I will be using the PGA to attenuate the signal to fit within the ADC range. Is that an acceptable plan of application? Please let me know if not.

    Also, back to my original question about the target black level code, I was asking for some tips on how to choose this value since this should be an input from the user that will be used in the auto black correction loop. Will it be safe to put it at full scale value? Or should a guardband be implemented with it. 

    Thanks,
    Paul

  • 0 in reply to Noniel Paul Novabos
    TI__Mastermind 24855 points

    Hi,

    Thank you for your feedback and your note. Sorry I put information for LM98620, not LM98714. Yes, LM98714 has 2.3Vpp. I think you may lose linearity or saturation for your 0.2V.  

    Let me review your question and feedback by 24th Feb.

    regards

    Shinya 

  • 0 in reply to Shinya Morita
    TI__Mastermind 24855 points

    Hi,

    Let me try to feedback "tips" based on datasheet description. If your input signal(image signal black to white) is 2.5Vpp full scale, I think we can not use it with full scale. CDS can accept 2.3V, however PGA min gain is 0.7V. And ADC max input range is 1.18V only(datasheet page8).

    2.3V x 0.7 = 1.61V. It is bigger than 1.18V.

    If you can reduce your input signal e.g 1.4Vpp instead of 2.5Vpp, you may be able to clamp your black level with around 2.0V, then set PGA gain x0.7. 1.4Vpp x 0.7 = 1.1Vpp

    Then signal at ADC 2.0V-0.9V.  Acc to datasheet page 8, ADC top reference is 2.07V and bottom is 0.89V. We still have extra room.

    Now we have room to use auto black target. 

    Thanks,

    regards

    Shinya  

  • 0 in reply to Shinya Morita
    Prodigy 200 points

    Hi Shinya,

    Thanks for the response. It is appreciated.

    Sorry I might've gotten confused on the specifications. It says in Page 7, Vin (Input Voltage Level) spec is 2.3Vp-p is that supposed to be Volts? It gave me an impression that it is a voltage range spec and so I thought my 2.5Vp-p CCD output would only need a bit of attenuation and clamping to fit within the ADC input range. 

    If so, would you please recommend an amplifier circuit to remedy my situation?

    Many Thanks,
    Paul

  • 0 in reply to Noniel Paul Novabos
    TI__Mastermind 24855 points

    Hi,

    Yes, page 7 VIN(Input voltage Level) SPEC is volts and 2.3Vp-p is SPEC.

    You may need external circuit to attenuation as you mentioned. We do not have recommendation for external circuit.

    regards

    Shinya  

  • 0 in reply to Shinya Morita
    Prodigy 200 points

    Hi,

    Thank you for your response.
    For an attenuator, do you think a simple resistor ladder (with precision resistors) would work well for this application?
    I am going for a simpler solution that would not use another IC. 
    I'm just afraid that there might be interactions with the input circuitry of LM98714 if I use a resistor ladder

    Regards,
    Paul

  • 0 in reply to Noniel Paul Novabos
    TI__Mastermind 24855 points

    Hi,

    I guess resistor ladder may have issue due to drive current capability limitation, however I do not know about resistor ladder use case. 

    Thanks,

    regards

    Shinya 

  • 0 in reply to Shinya Morita
    Prodigy 200 points

    Hi,

    I was thinking the same thing. Do you think the Source Follower buffer can be used to address this issue?

    Thanks,
    Paul

  • 0 in reply to Noniel Paul Novabos
    TI__Mastermind 24855 points

    Hi,

    As LM98714 Datasheet page 18, many customers have the emitter flower buffer before LM98714. Each customer designs it based on their sensor specification. I do not have other suggestion of attenuation by external circuit. Sensor output saturation level is determined by optical condition. I guess signal saturation level (Vpp) can be adjusted by optical condition.

    regards

    Shinya