analog ground and digital ground

isolate digital analogue ground

Hi

What is the difference between analog ground and digital ground?

In what conditions we have to short the analog ground and digital ground.

In what conditions we have to separate the analog ground and digital ground.

Thanks

Regards
nari

isolated digital ground from analog

In most cases, analog ground and digital ground will be shorted at one point. The purpose is to isolate those noisy digital ground from analog ground.

how to isolate analog ground and digital ground

if your pads are enough, analog ground and digital ground should be seperated; if your pads are not enough, analog ground and digital ground should be connected together at the pad, i.e. seperating them as much as possible.

analog and didgital ground should be short

For connection of analog and digital ground.Any how on board u have short both ground on board but u have various method.like star ground.Grounding at power supply connector.)Otherwise u can connect low value inductor on digital and analog ground.

short analog and digital ground togather

Different grounds in a circuit implies different potentials and different functions.
It is possible that parasitic effects like paracitic capacitors, paracitic resistance, small current overflow do not harm the digital part of the circuit as it works with specific levels of voltage or current and rejects all the other. Whereas in analog part these could destroy the whole circuit.

Therefore, designing different grounds for the two circuits you ammune one from the other and you make sure that you will not have the effects of the one interfear with the function of the other.
Of course this doesn't solve ALL your problems. Just some of them concerning coupling of paracitics.

D.

isolating digital and analog grounds

Your question is tricky. It applies many assumptions
(1) Digital ground and analog ground are already shorted if it is a common substrate, unless you have deep NWELL in process to isolate different grounds
(2) Even if u have common substrate, solid connection between them should be made as late as possible, like in PADs or in PCB level
(3) Anyhow, PCB will short them together, ppl always use ferite beads to separate grounds from one ground and then distribute to chips

hi guys,
i am using a ATmega8 chip along with its internal ADC to read an analog temperature reading from a thermocouple. As the chip has a single ended ADC input, i am forced to use the same ground plane for both the digital section and the analog signal conditioning section. Though i have used the star grounding technique in my pcb layout ( split grounds to the analog and digital section from the power supply), the microcontroller picks up a strong EMI/electrostatic noise from the thermocouple sensor through the analog section and the software flow goes haywire. I am using a simple non-inverting amplifier with an adequate LPF of around 10Hz at the input. Is using a instrumentation amplifier the only option left to minimise the noise? Or can i reroute the power supply rails ?
any suggestion would be highly appreciated.
thank you.
-----pundalik

p.s. the external noise picked by the sensor is generated from a blower and an ignition transformer ( spark plug).