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ADS8861: Direct compatibility between LMH32401 and ADS8861

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Replies: 28

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Part Number: ADS8861

Hi!

My design consists in detecting a light by a photodiode which I will place at the input of the LMH32401 device. At the output of the differential LMH32401 I will connect a differential ADC with SPI interface. Note that the ADC must be differential and also equivalent to a 100 Ohms load because according to his datasheet the LMH32401 is designed to drive at 100 Ohms load. Thank you for advising me an ADC who respect these conditions.

I chose ADS8861  but I am not sure that they are compatible directly, ie if I can connect them directly, knowing that the LMH 32401 is made to drive a load 100 Ohms. What do you think?

If you find that these two devices (LMH32401 and ADS8861) are incompatible, please advise me a differential TIA and a differential ADC with which I can realize my design. Note that I prefer to use differential devices to minimize common mode noise.

  • In reply to Keith Nicholas:

    Hello Keith,

    Thank you very much again for your answers.

    I have two more final questions, one clarification and one piece of advice.

    1. Questions:

    I understand that to use the full range of my ADC (ADS8861), I need to set Vocm to Vref / 2 (1.65V here), with full differential output from -Vref to + Vref. Is there no problem because the differential output range of LMH32401 is not as high? If this is really a problem, to get the most out of my ATI (LMH32401), can I set the Vocm pin to 1.1V?

    Can you explain to me from equation 1 of section 7.3.4 of the LMH32401 datasheet or another justification how you obtained the 0.75 V value that you recommend for VOD?

    2. Request for clarification:

    By removing R15, I also remove the link between the input and the output of the device OPA33. Is it true?
    By removing R24, should I connect the SS terminal of the REF6033 device to GND or leave it open?


    3. Request for advice:

    In my project, I will use my photodiode to detect lights in the 350 to 1100nm wavelength range, while keeping the same circuit. Can you suggest me a photodiode that is compatible with the TIA and ADC that I have already chosen?

    Finally, I want to inform you that I have already designed several circuits that are less complex than I will need for my project (power supplies, etc.) and I have always used IT devices because I am satisfied with them. This is why I keep asking you questions, so that I can still choose IT devices.

    Always counting on your usual collaboration and your quick response,

    Best regards.

  • In reply to Keith Nicholas:

    Hello Keith,

    I'm resending this email in case you haven't received it.

    Thank you very much again for your answers.

    I have two more final questions, one clarification and one piece of advice.

    1. Questions:

    I understand that to use the full range of my ADC (ADS8861), I need to set Vocm to Vref / 2 (1.65V here), with full differential output from -Vref to + Vref. Is there no problem because the differential output range of LMH32401 is not as high? If this is really a problem, to get the most out of my ATI (LMH32401), can I set the Vocm pin to 1.1V?

    Can you explain to me from equation 1 of section 7.3.4 of the LMH32401 datasheet or another justification how you obtained the 0.75 V value that you recommend for VOD?

    2. Request for clarification:

    By removing R15, I also remove the link between the input and the output of the device OPA33. Is it true?
    By removing R24, should I connect the SS terminal of the REF6033 device to GND or leave it open?


    3. Request for advice:

    In my project, I will use my photodiode to detect lights in the 350 to 1100nm wavelength range, while keeping the same circuit. Can you suggest me a photodiode that is compatible with the TIA and ADC that I have already chosen?

    Finally, I want to inform you that I have already designed several circuits that are less complex than I will need for my project (power supplies, etc.) and I have always used IT devices because I am satisfied with them. This is why I keep asking you questions, so that I can still choose IT devices.

    Always counting on your usual collaboration and your quick response,

    Best regards.

  • In reply to Andrianirina Rakotomalala:

    Hello,

    The output pins of the LMH32401 are limited from 0.39V to 2.87V max.  Setting the VOCM to 1.65V allows for the maximum swing.

    There are different combinations of VOCM and VOD that will likely work.  However, keeping everything centered (VOCM=1.65V, VOD=0.75V) provides the most margin.

    Equation 1 in the datasheet takes into account the output resistance (20ohm) and the nominal load resistance (100ohm).  In other words, the gain on the VOD pin is adjusted to give 0.75V output offset when VOD=0.75V, and RL=100.  If Rl is a different value from 100ohm, you can use this equation to calculate the proper VOD value to get a desired output offset.

    For your second question, yes, simply delete R15 resistor from the schematic.  (No component or connections)  For R24, leave the SS terminal REF6033 floating, or no connection.

    The below equations may help to better understand the relationship between the input current from the photodiode, the output voltage, and VOD.

    Vodiff-unloaded: The differential output voltage with no external load (Rl=infinity)

    Vodiff-loaded:  The differential output voltage with external load resistor Rl.

    Ipd: Photodiode current

    G: Gain, either 2k or 20k

    Vodiff-unloaded = 1.2*G*Ipd + 1.2*Vod

    Vodiff-loaded = Rl/(Rl+20)*1.2*G*Ipd + Rl/(Rl+20)*1.2*Vod

    If Rl=100ohm, then Vodiff-loaded simplifies to:

    Vodiff-loaded = 100/(100+20)*2.4*G*Ipd + 100/(100+20)*1.2*Vod = G*Ipd + Vod

    Regarding your photodiode question, I cannot provide any suggestions on this.  I suggest you contact the photodiode manufacturer for help on this subject.

    Regards,
    Keith

  • In reply to Keith Nicholas:

    Hello,

    Thank you for your answer and your explanation.

    Regarding the SS pin of the REF6033 device, its datasheet defines it as the short circuit current limit pin and recommends connecting it to a resistor to set the output short-circuit current limit, otherwise to connect it to VIN pin for highest current limit. Is it okay if left open?

    I understand your Vodiff-loaded equation = RL / (RL + 20) * 1.2 * G * Ipd + RL / (RL + 20) * 1.2 * Vod. In our case, what would be the value of RL because we have the ADS8861 as a load? Why do you say VOD = 0.75V is the right value?

    I'm really sorry to ask you all these questions because I would like to understand.

    Thanks for your patience with me.

    Best regards.

  • In reply to Keith Nicholas:

    Dear Keith,

    Continuation of my previous email.

    According to his datasheet, the short-circuit current limit (ISC) of the REF60xx family of devices is adjusted by connecting a resistor (RSS) on the SS pin. The short-circuit current limit when the REF60xx device is sourcing current can be calculated by using the Equation 4.

    Also, the short circuit current limit when the REF60xx device is sinking is calculated by using the Equation 5.

    Finally, the recommended output current of the REF60xx also depends on the resistor connected to the SS pin. The recommended output current (sourcing and sinking) for the REF6025, REF6030, REF6033 and REF6041 is given by Equation 6.

    So, unless I am mistaken, we cannot leave the SS pin open. Please correct me if I am wrong and explain to me why.

    Please answer me today if possible.

    Best regards.

  • In reply to Andrianirina Rakotomalala:

    Hello Andrianirina,

    Good catch!  You are correct about the REF60xx family.  This pin does need a resistor; a typical value for the resistor is 120kOhm.

    Regarding Vod, the maximum input current range with G=20k is 0uA to 75uA.  This translates to an output differential voltage from 0V to 1.5V (with external Rl=100ohm).  The input range of the ADS8861 is +/-Vref, so in general, you can set the differential voltage to zero volts at 1/2*Input range.  In this case, 1/2*1.5V=0.75V.

    However, since the input range of the ADS8861 will be +/-3.3V, you could connect VOD to ground, VOD=0V, and then the output differential voltage will range from 0V to 1.5V, which is still inside of the ADS8861 input voltage span.  The minimum output voltage on the LMH32401 is about 0.4V.  This will set the minimum VOCM voltage to VOCM=1.5/2+0.4=1.15V.

    Regards,
    Keith

  • In reply to Keith Nicholas:

    Dear Keith,

    I am not tired of thanking you yet.

    In conclusion for, I therefore connect the VOD pin of LMH32401 to the GND and I apply a voltage of (Vref / 2) = 3.3V / 2 = 1.65V on the VCOM pin. Am I right?

    Thank you for responding to me today.

    Best regards.

  • In reply to Andrianirina Rakotomalala:

    Hello Andrianirina,

    Yes, for your use case, that will work.  VOD=0V, VOCM=1.65V.

    Regards,
    Keith

  • In reply to Keith Nicholas:

    Dear Keith,

    I am coming back to you to be sure of what we decide.

    Won't connecting VOD to GND reduce the output generated by the LMH32401 to half?

    If I am not mistaken if we keep the VOD pin open, the device will provide 510mV to the VOD pin internally. Why not leave it VOD open?

    Why did you give up connecting VOD to a voltage of 0.75V?

    Finally, are you sure that VOCM = VREF / 2 = 1.65V is not too high compared to the output level of LMH32401? So this is better compared to VCOM left open, in which case the default will be 1.1V?

    Please anwer me urgently.

    Thank you so much.

  • In reply to Andrianirina Rakotomalala:

    Hello Andrianirina,

    Since you are using less than 1/4 of the full scale input range of the ADS8861 (6.6Vpp full scale input), with the output of LMH32401 limited to 1.5Vpp, you have multiple options on where to set VOCM and VOD.

    In any case, if you want to set everything in the 'middle', where you have the most margin, then setting VOCM to 1.65V (optimized for the ADS8861 input range) and VOD to 0.75V (Optimized for the LMH32401 internal bias point) will achieve this.

    Regards,
    Keith

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