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THS3217: Power supply vs Harmonic Distortion

Anton Ivanov2
Prodigy 190 points

Part Number: THS3217
Other Parts Discussed in Thread: LMH6401,

Hi!

I need to drive LMH6401 with THS3217 and get very low HD2 and HD3 (not worse as datasheet's one at +6V and -6V supplies)/

There is a graphic of HD2 and HD3 vs power supplies in datasheet. 

 

I need to get characteristics of HD2 and HD3 as in datasheet for D2S+OS for +6V and -6V supplies, but it's a real problem for me to get -6V, but easy to get +7V. So, my question is would i get the same characteristics as in datasheet for +6V and -6V if i use +7V and -5V (non-simetrical supplies).

it is also said about D2S: "The D2S external output is loaded with a 200-Ω resistor to ground. Lighter loading on the VO1 pin (versus the 100 Ω used to characterize the D2S only) results in increased frequency response peaking. Heavier loading degrades the D2S distortion performance"(page 47). Does it mean, that if i want to get single-ended signal from differential signal without any gain, I can use external output (VO1 if D2S with 100 Ohm load to get better HD2 and HD3 then if i use this external output with 200 Ohm load?

At the same time figure 67 shows shows the output network used to emulate a 200-Ω load resistance (RLOAD) while presenting a 50-Ω source back to the D2S output pin. It looks like it's not a good idea to load VO1 pin just with two 50 Omh resistors, isn't it? And it's ussually the bigger resistors the better HD2 and HD3. 

There is my scheme:

THS3217_D2S.TSC

Could you please help me with these questions or give an advice how to deal with supply, if i want to use nonsimetrial voltage supplies as in the project (-5V and +7V) and save HD2 and HD3 as in datasheet  for +6V and -6V voltage supplies?

Thank you very much!

Anton

  • 0
    Guru 48395 points

    So Anton, if you have a differential or complementary input (why you would you be using THS3217), why are you going single ended there, then back into the differential LMH6401 DVGA/ 

    You distortion questions will be very difficult to answer, also keep in mind there is part to part variation in HD2 particularly, less so in HD3. 

  • 0 in reply to Michael Steffes
    Prodigy 190 points

    Hi, Michael.

    I have differential signal from DAC, i need to control attenuation and gain so i use LMH6401. But then i need to get a single-ended signal, so i use THS3712 as load for LMH6401 to convert differential signal to single-ended.

    Anton

  • 0 in reply to Anton Ivanov2
    Guru 48395 points

    That sequence makes more sense, 

    You can also maybe use the DAC reference current to scale the signal vs the LMH6401

  • 0 in reply to Michael Steffes
    Prodigy 190 points

    If i'm not wrong, the scheme doesn't know anything about voltage. It doesn't matter if i use single 12 V or two supplies -6V and 6V, one differece is that i need to set the working points for each amplifier. It's easy to do it for D2S with connection  VMID_OUT with VREF, but then i must use capacitor between output of D2S and input of OPS using external output and set the working point of OPS with connection VINP to +V through a resistor, that forms a voltage divider with internal resistor 18,5 kOm. Then if i calculate right i gets 1 V working point on the OPS output.

    There is my scheme in TI:

    THS3217_D2S_1.TSC

    Am i right? 

    Anton

  • 0
    TI__Genius 15605 points

    Hello Anton,

    In regards to your first question, yes it should be fine to use non-symmetrical supplies in regards to the harmonics characteristics. However, the supplies of each stage need to be the same: +VCC1 = +VCC2 & -VCC1 = -VCC2.

    For the second question I think there is a misunderstanding in the passage of the datasheet you quoted. When "lighter loading" is mentioned this means using a larger resistor so that there is less loading on the amplifier. The lighter loading in this case is using the 200-ohm resistor instead of the 100-ohm. So your statement is correct that larger resistors typically have better hd2 & hd3 performances.

    Note that the above statement is in regard to the D2S stage, in terms of the OPS stage there is actually improved distortion performance at 100-ohm load. The datasheet states, "The distortion performance is extremely robust as a function of RLOAD (see Figure 33 and Figure 34). Normally, heavier loads degrade the distortion performance, as seen for the HD3 in Figure 34. However, the HD2 actually improves slightly going from a 200-Ω load to a 100-Ω load."

    Best,

    Hasan Babiker

  • 0 in reply to Hasan Babiker31
    Prodigy 190 points

    Hi, Hasan!

    Thank you for your answer.

    Could you please tell me about another one thing, that i can't understand. 

    If i use single supply +12 V, it means that voltage on the pin VMID_OUT will be +6V and that would be great for the first stage: D2S. But there is will be +6 V on the output of D2S and It means, that there will be +6 V on the input of OPS. OPS will try to gain this +6V and will get +max on it's output, not average +6V as we need. What i do wrong? Should I use external output of D2S and connect it with external input of OPS with capacitor and get +6V with resistor devider?

    I see in TI that if I use +12 V, i get +10 V (output is in cutoff) on the OPS output, but i need average +6 V?

    I mean i understand how works first stage with it's VMID_OUT and V_REF, but i can't understand how works second stage OPS if i use single supply. How is working point setted?

    Best wishes,

    Anton

  • 0 in reply to Anton Ivanov2
    TI__Genius 15605 points

    Hello Anton,

    You can use Figure 103 as an example of how to use the device in single supply operation. The main thing to note here is all DC signals need to be referenced to mid-supply including the OPS. Gaining the 6V input should only be a concern if you use a grounded VMID_OUT rather a 6V DC mid-supply.

     

    Best,

    Hasan Babiker

  • 0 in reply to Hasan Babiker31
    Prodigy 190 points

    Thank you, Hasan!

    I tried your scheme and it didn't work in Tina, so it made me to hasitate if i were right. It looks like a model doesn't work properly with +12 V, it works only with dual-supply +6V and -6V. When i try your scheme i really get +6 V on outputs of D2S and OPS, but my signal becomes very very small... 

    Best wishes,

    Anton.