Isolating Analog and Digital GNDs
Isolating Analog and Digital GNDs
(OP)
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!
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!





RE: Isolating Analog and Digital GNDs
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.
RE: Isolating Analog and Digital GNDs
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.
RE: Isolating Analog and Digital GNDs
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.
RE: Isolating Analog and Digital GNDs
I do agree with tpavlics explanation of why this might be used.