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Questions on CFB Capacitor

AVINASH KOUSHIK
Prodigy 180 points

Hello,

I am in the process of designing my own coil for about 1um resolution. After obtaining the tank circuit, another important parameter which needs to be changed is the CFB Capacitor. However I am unable to obtain proper methods of fixing this capacitor for a changed tank circuit. The datasheet and other discussions in the forum mention that on the CFB pin, a 1V(p-p) is required for the stability of the data & in order to obtain it we need to keep changing the capacitors between those pins and observing the CFB waveform which is highly infeasible . Any Ideas on how to solve this problem ? Also, in the default coil with the LDC1000 EVM, I use a passive probe to test for the signal on the CFB pin and the signl was continuously wobbling, not giving a stable 1V(p-p) waveform. I also tried with a 1kOhm resistor at the input, but with no improvements. Any reason for this ?

With Regards,

Avinash

  • 0
    TI__Expert 4890 points

    Hi,

    You just need to keep the signal in CFB at about 1Vpp(When target is far away from the coil).

    Regard,Sunny.

  • 0 in reply to Sunny Qin
    Prodigy 180 points

    Yes, I know that. But I am only saying that I am unable to achieve it with a passive probe (10:1 , by agilent). I wanted to know how I will be able to achieve a proper 1Vp-p on the CFB pin using this. Secondly when  construct my own coil instead of the default one, how can I change the CFB capacitor.

    With Regards,

    Avinash

  • 0 in reply to AVINASH KOUSHIK
    TI__Expert 4890 points

    Hi,

    1, What do you mean by saying:  I wanted to know how I will be able to achieve a proper 1Vp-p on the CFB pin using this?

    2, You can refer to the datasheet: Page 13.

    Regards, Sunny

  • 0 in reply to Sunny Qin
    Prodigy 180 points

    Hello,

    I do not think you are understanding my question , all I wanted to know was since the 1Vp-p on the CFB pin is a proper indicator of the stability & when the coil is changed from the default to a custom made, I need to change this capacitor. The method on the page 13 of the datasheet says that we need to keep changing the capacitor until the 1Vp-p is achieved without mentioning any approximate formula for choosing this capacitor. So it becomes cumbersome to change this capacitor everytime until a proper signal is achieved. So I just wanted to know if there are any proper methods available for choosing the capacitor.

    With Regards,

    Avinash

  • 0 in reply to AVINASH KOUSHIK
    TI__Expert 4890 points

    Hi,

    As showed in the datasheet, you needn`t to change the caps all the time,  for example, if a 100pF filter capacitor is applied and the signal observed on the CFB pin has a peak-to-peak value of 200mV, the desired 1V peak-to-peak value is obtained using a 200mV / 1V * 100pF = 20pF filter capacitor.

    So you may need to change only two or three time at most, as the parameter of the coil is unsure, so it difficult to calculate the caps.

    Regards, Sunny

  • 0 in reply to Sunny Qin
    Prodigy 180 points

    Hello,

    Thanks a lot. Yes that is correct, but my only concern was that I might damage the pads at the CFB pins. Also I do not have an Active probe & hence the signal keeps wobbling. I tried using a 1KOhm resistor across the pin, but it would not work. Any way of solving this?

    With Regards,

    Avinash

  • 0 in reply to AVINASH KOUSHIK
    TI__Expert 4890 points

    HI，

    Don`t worry about the wobbling, use 1kohm resistor across the pin, change the caps until the signal on the CFB is about 1Vpp will be good, just about 1Vpp will be ok.

    Regards, Sunny.