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Design >> Analog Design >> Nature of Bandgap Noise and Ground Bounce
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Message started by Sumit Adhikari on Aug 13^th, 2009, 1:52am

Title: Nature of Bandgap Noise and Ground Bounce
Post by Sumit Adhikari on Aug 13^th, 2009, 1:52am

Hello All,
What could be the nature of Bandgap Noise?. Also What could be the nature of Ground Noise? (I think it would be difficult to estimate the nature of the noise). Anybody analyzed it experimentally ?
Regards,
Sumit

Title: Re: Nature of Bandgap Noise and Ground Bounce
Post by ci on Aug 18^th, 2009, 4:13pm

Ground bounce originates from transient currents flowing through the power distribution impedance. Simulation tools typically embed ground bounce into supply noise giving the false impression that the ground net is clean and only the supplies (positive and negative) move with the respect to the ground. Postprocessing of simulation results can further extract/compute the ground bounce noise.

Ground bounce may affect the bandgap output since this output is a voltage referenced to ground. Ground electric potential changes across the circuit and these differences may couple into the bandgap reference distribution circuitry.

Cosmin Iorga, Ph.D.
NoiseCoupling.com
http://www.noisecoupling.com

Title: Re: Nature of Bandgap Noise and Ground Bounce
Post by Sumit Adhikari on Aug 18^th, 2009, 6:24pm

Can you please tell me the distribution nature of ground bounce.
Thanks for your concern.
Regards,
Sumit

Title: Re: Nature of Bandgap Noise and Ground Bounce
Post by ci on Aug 20^th, 2009, 12:28pm

If I understand correctly what you asked me, ground is typically distributed as a plane or mesh across all circuits. In both cases there is a 2-dimmensional distributed impedance associated to the ground (and to the power supplies as well). This impedance results from the finite resistance of the conductive material and the loop inductance. Ground current flow has also a 2-dimensional distribution, and due to V=IR drops it creates an electric potential distribution. Dynamic ground currents generate dynamic electric potential distribution, so each device connected to ground may see a different potential than the expected zero Volts.

Cosmin Iorga, Ph.D.
NoiseCoupling.com
http://www.noisecoupling.com

Title: Re: Nature of Bandgap Noise and Ground Bounce
Post by Sumit Adhikari on Aug 21^st, 2009, 12:13am

Thanks for the reply.
Can you please tell me the distribution type. normal, uniform etc.
BR,
Sumit

Title: Re: Nature of Bandgap Noise and Ground Bounce
Post by loose-electron on Aug 22^nd, 2009, 2:39pm

You can not generalize it. It will be unique to the circuits and their layout.

Title: Re: Nature of Bandgap Noise and Ground Bounce
Post by Sumit Adhikari on Aug 22^nd, 2009, 8:00pm

Thanks. But if even when the cirecuit is not yet designed nor the layout
is done, how must I choose my random numbers to see the effect of
ground bounce. What do you suggest ?
Regards,
Sumit

Title: Re: Nature of Bandgap Noise and Ground Bounce
Post by RobG on Aug 24^th, 2009, 8:40am

ci wrote on Aug 18^th, 2009, 4:13pm:

Ground bounce may affect the bandgap output since this output is a voltage referenced to ground. Ground electric potential changes across the circuit and these differences may couple into the bandgap reference distribution circuitry.

There is another more insidious effect. AC currents in bipolar are nonlinear, so the DC value of the current changes with the amplitude of the AC current. The net effect is that the DC output of the bandgap will change depending on the amplitude of the AC noise. The AC current can be coupled in from the base/collector capacitance of the PNP, or from other sources. This is in addition to the effect you describe. It is frequency dependent.

Many an engineer have freaked out upon seeing their dear bandgap voltage drop 50 mV when the noisy circuitry is switched on.

Title: Re: Nature of Bandgap Noise and Ground Bounce
Post by ci on Sep 3^rd, 2009, 1:03pm

Sumit,

You have asked: "But if even when the circuit is not yet designed nor the layout is done, how must I choose my random numbers to see the effect of ground bounce. What do you suggest ?".

Before designing your circuit and creating the layout you can only estimate the ground bounce based on various info you have available. You can search for some papers covering early estimation of supply/ground bounce, or you can read chapter 8 of the book "Noise Coupling in Integrated Circuits: A Practical Approach for Analysis, Modeling, and Suppression". There are also some low cost tools that can help you here; one of them being ChipQuake.

Cosmin Iorga, Ph.D.
NoiseCoupling.com
http://www.noisecoupling.com