I have a power supply board that supplies a DSP, FPGA, CPLD and other components of a system. When I measure the resistance between power rails and GND using a multimeter I get the following readings:
- Between 12V and GND: Infinite
- Between 5V and GND: Infinite
- Between 3.3V and GND: 250 Ohms
- Between 1.2V and GND: 40 Ohms
The 3.3V and 1.2V rails have many decoupling caps around power supply pins of the DSP and FPGA, are they the only cause for this much reduced isolation between these voltages and ground?
Is it safe for the long run to work with these specs? The system is now running perfectly.
Also how can I avoid this issue in future designs?
Edit #1
In a project I'm currently working on, one of the tests that will be done on the product will be isolation tests. The customer will measure the resistance between supply rails and Chassis ground and want it to be greater than 10 Mega Ohms, that's why I want to avoid these low resistance values. Is there a device used to measure isolation? or it's done with a normal ohmmeter?
Edit #2
These measurements are done when the system is powered OFF.
Best Answer
When I've heard about power supply "isolation testing" before, the context has been a test done on the mains inputs to a system or device. I've also heard these tests called "insulation resistance" tests or "hipot" tests, though those could be slightly different tests. The test confirms that voltage applied to the mains input won't connect to the case of the UUT and cause a shock hazard.
The test involves applying a fairly high voltage (1 to 5 kV or so) to verify there's no spark gaps or insulation breakdown paths between the mains and user-accessible surfaces.
According to this, isolation testing tests
The main point is that this is a test you would do on the mains input of an isolated power supply. It is not a test you should be doing on the inputs where your operating circuit expects to receive its local regulated input power.