ISO124U power requirements

Hello Bill Pierce,

1) The advantage of using +/-15V over +/-12V is a wider dynamic range.

2) The size of the DC/DC converter needed depends not only on the quiescent current of the ISO124 but also on the load connected to its output. The ISO124 can indeed drive +/-15 mA typical, but the output current is actually determined by the load. At a very minimum you need to supply quiescent current plus peak load current (peak output voltage divided by load connected to output).

Hope this helps and let me know if I can be of further assistance.

Best regards,

Jose

Jose,

I'm sorry for the slow response. I had to go out of town for a couple of days unexpectedly.

Since my inputs are limited to +/-10V, I can't see that the wider dynamic range of +/-15V power is really any benefit.

Regarding the power requirements, since the ISO120 has 2 isolated sections that must be provided isolated power. I really need to size the DC/DC converters for both the input and the output section. I don't yet have all the details of the circuit that my card will be driving, but it should be a fairly high impedance, at least 10K. So, the output current from the ISO124s should be rather small. The frequency of my inputs should be 1KHz or less. I'm hoping that means that the input side will not require any more power than the output side.

In figure 12 in the ISO124 datasheet a pair of DCP011515DB/DCV011515D isolated unregulated DC/DC converters are used to power a single ISO124 with the annotation that 20mA is available from each rail as auxiliary power. This implies that a ISO124 needs no more than 12mA per rail, assuming a lightly loaded output. I have found a number of isolated regulated (I'm not comfortable using unregulated supplies on my card) DC/DC converters that can provide up to 200mA per rail which would allow each of my 8 ISO124s to draw up to 25mA which should be more than adequate. Once we get the prototype built, we can reassess the power requirements and possibly change to a less powerful DC/DC converter to save some money.

Thanks for your help. I'm working on this project solo and it's nice to be able to run this by someone else. I find that stopping to explain what I'm trying to accomplish often helps me to better organize my thoughts.

Bill Pierce

Huntsville, AL

Bill,

Thanks for you interest in our analog isolated products and no apologizes required.

I read that your inputs are +/- 10V. I understand that you are using a +/- 12V supply and you are concerned about possible clipping with the ISO124. If you read the data sheet both the output and input ranges are specified to a +/- 10 V range with power supplies of +/- 15 V. This is a very dependable specification because it is tested on the production floor before shipping. You will also notice that the typical I/O ranges are +/- 12.5 V. If you count on these typical (untested) specifications (which are 2.5 V below the supply), you will frequently see clipping at the higher and lower end of your input signal and/or output signal.

With your proposed power supply of +/-12 V you can really only count on an input/output swing of +/- 7 V.  

About your driving concerns, if you are trying to keep you DC/DC converters as ‘small’ as possible it is really important that you understand your output requirements. You will find on the input stage, the power dissipation will not vary that much.