Isolating Analog and Digital GNDs 1

[tdbeng]

What is the purpose of placing an inductor between analog and digital grounds? I've seen this done several times in data acq. and control circuits.

For example, I have this circuit on a single PCB that uses +24VDC and +5VDC, which come from different switcher supplies. The +24VDC is being used to control several brushed DC motor (w/ H-bridge ckts). The +5VDC is used for CMOS logic and an embedded PC/104. I've always tied the grounds together at a single point on the PCB. Is it common to use an inductor between the two?

Thanks to anyone who can make this clearer!

 

[tpavlic]

The digital ground is typically pretty noisy. The high frequency digital signals cause quite a commotion on the digital ground. You typically want your analog ground to be as "flat" as possible and as "still" as possible. If it is directly tied to the digital ground, this can cause your analog ground to start to feel the movement of the digital ground.

However, you probably do want the two tied together at DC. You want them both to be sitting at the same potential.

If you connect the two with an inductor, this prevents high frequency garbage from flowing onto the analog ground, yet it still allows the two to be DC coupled together.

 

[GlennAnalog]

I question the use of the inductor. Most mixed signal ICs bring out separate digital and analog grounds simply to keep the digital current from flowing through the same bond wire as the analog section, thus creating a common mode noise.

Placing an inductor between the two grounds could create an L di/dt voltage drop that could potentially damage the mixed signal IC; at the very least be the source of bad data.

Analog Devices has some good application notes:

AN-345 Ground for Low and High Frequency Circuits

Burr-Brown (TI) and Linear Tech also have some good notes.

 

[Lewish]

On the other hand, Brooktree, you know the guys who did video decoder chips, required an inductor between the 2 grounds or the part would not work.

It all depends on the individual design and the I.C.s being used.

I prefer high frequency ferrite chips instead of inductors. Murata makes one, that most video boards for PCs use. It is impossible to get if you aren't doing very high volume. But, there are good alternative for less stringent uses.

 

[buzzp]

I would also question the use of an inductor in most applications. Inevitably, this will cause a potential difference between the two grounds. Depending on the circuitry, this may cause erroneous readings of any measurements.
I do agree with tpavlics explanation of why this might be used.