What is an Isolated Analog Input Module? 2

[123MB]

Hi All

I am interested to know what you think an 'isolated Analog Input module is' as I have not been able to find a consensus on the meaning of this term among manufacturers and literature.

1) Some literature I have seen describes an isolated analog module as one where the analog common is isolated from power supply/chassis/earth ground.

2) However, most cheap analog input modules (i.e. 1769-IF8) are such that the analog common is isolated from power supply/earth/chassis ground, but these are not advertised as isolated analog input modules. On the 1769-IF8 there also appears to be isolation between each group of 4 channels, but there is no channel to channel isolation.

3) Alternatively some cheap analog input modules (i.e. Siemens 6ES7331-7KF02-0AB0) are advertised as an isolated module, however, as per the 1769-IF8 above, they only have power supply isolation and group isolation, no channel to channel isolation. Each group of 4 channels shares an ADC and hence there are common mode voltage restrictions.

So 2) and 3) above are both technically the same, electronically, but one is advertised as an isolated module and the other is not.

4) And then there's the high end Isolated Analog Input modules (i.e. 1769-IF4I) which have channel to channel isolation, and channel to power supply/chassis isolation, with a separate isolation amplifier and ADC per channel.

So what is an Isolated Analog Input Module?

Is an AI module isolated if the ACOM is isolated from power supply and chassis grounds? Surely this would include most AI modules

Or is an AI module isolated only if it has isolation between all channels, and to power supply/chassis? This would include only the top of the line expensive low density modules?

Thanks

Regards,

Michael.

 

[danw2]

>So what is an Isolated Analog Input Module?

Exactly what you’ve laboriously discovered in your research: whatever degree of isolation is designed into the product.

My experience is that there’s no free lunch sothat you get what you pay for.

 

[123MB]

Indeed. Thankyou.

 

[Skogsgurra]

There used to be four categories:
Not Isolated. With Common and Ground tied together.
Isolated, where Common is isolated from Ground.
Differential, with + and - inputs semi-isolated (megohms to Ground)
Isolated, with + and - having Giga-ohm or Tera-ohm to ground with the aid of galvanic (opto or transformer) isolators.
One may also put flying capacitor isolation in the latter category.

All isolation is about DC resistance. When an AC common-mode voltage exists, it is also very important that the CMRR is adequate. That is usually a problem with most applications where PWM residues or other HF signals make their way into the signal path. Mostly through the signal cables or because the DUM (Device under measurement) has an HF potential on its ground or signal ground potential.
Warning, a CMRR in the 60 - 80 dB range seems to be OK. But it is usually given at 1 kHz and doesn't do any good when the common-mode signal is in the MHz range, which is often the case with PWM residues.

Other terms are Two-port and Three-port isolation, which I think is self-explanatory.


Gunnar Englund

http://www.gke.org

--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

 

[123MB]

I think, in most cases, when people refer to a non-isolated analog input module, they are referring to one that uses the ground refernce/chassis ground as analog common.