What's the use of "AC coupling" in a meter? At what minimum frequency do you expect the high pass filter to occur, and how steep is the roll off? I don't want a meter that works that way, and I am surprised to learn that such a meter even exists.
For measuring ripple for example, like Bloch stated
. Note that the R in this RC filter is usually 1 Mohm, so even with 100 nF of capacitance the -3 dB point is already at ~1 Hz. Note that DC mode is acting as a low pass filter, too. NMRR rolls of at low frequencies. At 1 Hz signal will probably be recorded as a fluctuating DC signal. Most of the meters out there seem to be AC coupled and some of the fancy ones have AC + DC as an option. This is probably the only option for average-responding meters, since a DC voltage is not going to go well through that precision rectifier (unless they make it a bridge rectifier). AC + DC is usually only available on true RMS meters.
Furthermore, if you want to measure true RMS you must consider all frequency components down to DC or you are getting a false reading. If the meter can display AC+DC separately that's good, but if the meter can only show one value, I want to know all the current flowing in my wire, or at least have a warning that there is a DC component the meter is hiding from me.
Agreed that in some cases (like an unknown mix of AC and DC), AC+DC is preferable, which is why some meters offer that function
. In many cases, you will have well defined AC and DC components, however. For example, you might have a +3.3 VDC supply rail with 100 mV of 60 Hz ripple, plus some high frequency hash that's probably beyond the meter's bandwidth.
Home
Help
Search
Login
Register
