[SI-LIST] Re: nonlinear in power ground noise of clock driver

  • From: Ravinder.Ajmani@xxxxxxxx
  • To: si-list <si-list@xxxxxxxxxxxxx>
  • Date: Thu, 4 Dec 2003 09:16:27 -0800

A solid Tantalum capacitor has ESR of several hundred milliohms, whereas 
ESR of a Ceramic capacitor is about 10 -20 milliohms.  Perhaps this is 
what makes such a large difference in the noise reduction properties for 
the two types of capacitors.  Another experiment you can try is to replace 
the Tantalum capacitor with a Polymer Tantalum capacitor, which has ESR 
below 100 milliohms.  If this causes significant reduction in power supply 
noise then ESR is the key factor.
Regards, Ravinder
Server PCB and Flex Development
Hitachi Global Storage Technologies

Email: Ravinder.Ajmani@xxxxxxxx






Zhangkun <zhang_kun@xxxxxxxxxx>
Sent by: si-list-bounce@xxxxxxxxxxxxx
12/04/2003 03:50 AM
Please respond to zhang_kun

 
        To:     si-list <si-list@xxxxxxxxxxxxx>
        cc: 
        From:   si-list-bounce@xxxxxxxxxxxxx
        Subject:        [SI-LIST] nonlinear in power ground noise of clock 
driver




Dear All:
Recently, my colleague found out something interesting about clock driver.

In our product, CDC2516 of TI is used as clock driver. The clock frequency 
is 100MHz. The power of CDC2516 is isolated by one bead. There is some 
local decoupling caps as following:
    several 0.1uF or 0.01uF ceramic cap
    one 10uF tan cap

We measure the power ground noise of CDC2516. The power ground noise is 
about 700mV. The spectrum of the noise is at 100MHz, 200MHz, 300MHz, and 
so on.

We use several method to decrease the noise. Theere is no use.

My colleague change the 10uF tancap to one 10uF ceracap(X7R). The noise 
decrease to 17mV. I am a little confused. I have simulate the impedance of 
decoupling circuit. There is no difference in frequency domain beyond 
30MHz between using 10uF tancap and 10uF ceracap. According to my 
experience, the power ground noise should not decrease.

I think there is something nonlinear. I am not familar with the circuit of 
clock driver. There is no SPICE model of CDC2516 in TI website. Therefore 
I could not simulate the power ground noise.

Could you give me some advice?

Best Regards

Zhangkun
2003.12.4


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