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Design >> Analog Design >> Substrate connection in a mixed signal chip
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Message started by vijay on Nov 15^th, 2005, 12:40am

Title: Substrate connection in a mixed signal chip
Post by vijay on Nov 15^th, 2005, 12:40am

Hi,
My doubt is in a mixed signal chip, if we have both Analog and Digital ground,where should I connect my bulk and why?
If i connect the bulk to analog ground, then large substrate noise current should flow through the analog pin modelled as an inductor (the bond wire inductance).
If i connect the bulk to digital ground then there will be large voltage variations in the bulk voltage due to the digital circuit.

So where exactly do i connect my bulk.Should I connect my bulk to both the grounds!!!!!!!!

Thanks in advance,
V

Title: Re: Substrate connection in a mixed signal chip
Post by Paul on Nov 15^th, 2005, 12:00pm

Vijay,

basically you must
1) avoid injecting additional noise into the substrate
2) pollution of your clean ground with substrate noise.
Because of (1), you cannot use the digital ground for substrate connection, because of (2) you cannot use the analog ground either. Ideally, you use two clean (separate) ground pads, one for the substrate noise pickup near the noisy part and one for the substrate biasing near the analog part.

Notice that your last suggestion would mean connecting both ground nodes together, creating a direct (low-ohmic) path for switching noise on the digital ground to the analog ground.

Paul

Title: Re: Substrate connection in a mixed signal chip
Post by ywguo on Nov 18^th, 2005, 11:17pm

Vijay,

Paul has propose the two basic rule. In general, how to assign power supply and ground depends many factors including the requirement on performance, the topology of the circuit, the available pin resources, and so on.

If possible, provide one clean (seperate) ground to analog circuit or as the guard ring for analog circuit. For some very sensitive design, assign a seperate ground for bulk of the digital circuits to prevent the substrate from pollution.

Best regards,
Yawei

Title: Re: Substrate connection in a mixed signal chip
Post by Sezi on Nov 23^rd, 2005, 10:40am

Hello,
I'm a little confused here, do we connect ground of analog circuit to a clean pad, ground of digital part to another clean pad and substrate to a 3rd pad?

Title: Re: Substrate connection in a mixed signal chip
Post by Paul on Nov 23^rd, 2005, 12:28pm

Hi Sezi,

what I tried to suggest was the following:
- one or more pads for analog ground (usually quite clean thanks to constant power consumption)
- one or (usually) more pads for digital ground (usually quite noisy due to switching noise)
- one clean pad for substrate biasing close to the analog section
- one clean pad for noise pick-up from the substrate close to the digital part.
Of course the latter will not remain clean once it is connected to the substrate and picks up the noise. What I mean by clean in that case is that it should not be noisy by nature, otherwise it will inject additional noise into the substrate.

As Yawei points out, this is an optimum solution which requires some overhead (pads, routing). It is not always needed and simpler intermediate solutions are frequently sufficient.

Paul

Title: Re: Substrate connection in a mixed signal chip
Post by Sezi on Nov 23^rd, 2005, 3:12pm

Thanks Paul now i got it ;D