12-02-2014, 06:06 AM
(11-02-2014, 03:10 PM)pwdrive Wrote: What would be the DC offset of a 5 volt pulse sitting on a common rail of zero volts.
Rather than DC offset think of it as the equivalent DC value. There is no such value for a non-periodic event (think about it) but generally the solution here is to calculate the RMS value which is simple enough with an opamp and a few oyher components or a dedicated chip (e.g. AD8436) - its the integration over time of the area under the curve with respect to some reference point, typically 0V. In practice this is generally done over a few cycles of the input and continuously recalculated using a sliding window i.e. it's approximated over a set time constant.
The simplist way to think about the RMS value of a waveform is that it is the DC equivalent of the waveform such that when applied to a pure resistance it produces the same amount of heat as the waveform under test. Indeed, this is exactly how early true-RMS meters worked... they measured how hot a resistor (or equivalent) became when the arbitrary waveform was applied and compared that with values measured when heated by a known DC value (there are a number of ways of doing this)...
Most modern cheap DVM/DMMs which purport to be "RMS" or "true-RMS" meters (both should be the same really) do no such thing as
- they are AC coupled so ignore any DC component
- they don't integrate the input properly anyway (they assume a sine wave)
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