How to connect the grounds as STAR GROUND and How to know cross check them?

How to connect the grounds as STAR GROUND and How to cross check them?

Hai,

I am little bit confused about the routing/wiring to form a star ground.
I have a designed a signal averager (box car averager) circuit with few digital ICs (10 numbers) 74ls00, 74ls04, 74ls244, 74ls393 and analog ICs (26numbers) (like lF356 and LF398 e.t.c). I have made two individual grounds for those type and called as analog ground (AGND) and digital ground (DGND). The empty space at the bottom side of the PCB is mostly copper pour connected to AGND where as DGND has no plane ground on PCB. Also I am using +5 Volts and DGND and +12V and -12V and AGND. The respective grounds are isolated on PCB. Each ground has its own termination point on PCB. The intention for having two grounds is to minimize the noise. I think this can be achieved if we connect them as star ground. I made a regulated power supply of +5V, +12V and -12V with ground common. I connected this common ground directly to the chasis. I have connected the AGND and DGND terminated point on PCB to the chasis(used as main ground).

I observed the resistance between any ground on PCB to the chasis is less than one ohm. But If switch on the power supply, I found the reisistance between the analog ground AGND and chasis is 1.6 ohms and digital ground DGND is 10 ohms. And the potential with respect to each other is not at zero.

Now let me know is this network is star grounded. How to check whether it is star grounded correctly.

After star grounding will the resistance among the the grounds will change before and after power supply. Let me know the ideal case also please.

With thanks and regards

DRKJREDDY
India

Pls keep large current path short and straight. Then connect small-signal ground to true ground by star connection with narrow wire.

Re: How to connect the grounds as STAR GROUND and How to cross check them?

leo_o2 said:

Pls keep large current path short and straight. Then connect small-signal ground to true ground by star connection with narrow wire.

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kindly let me know resistance between grounds before and after switching the power supply on. Will the resistance between the grounds vary.

For ground bounce consideration, we suppose it doesn't vary.

Hello

Thanks for your reply. I found the potential difference and analog ground is 20 mv. Is there necessary to make it near to zero millivolts difference between them.

Of course, smaller difference is better. However, it is hard to be zero.

The star gnd point should be on the PCB where the AGnd and DGnd are connected. This may require the +5 and +/- 12 supplies to have seperate grounds.

could you kindly send the images of noise with and without star ground and in ground loops configuration (if you have).