Electronics > Beginners
Why are V(rms) measurements frequency dependant?
Kleinstein:
A rather good OP for AC amplification would be the OPA140 (often the cheaper version OPA141 will also work). Noise should be about 6 times lower than the max4239 and no extra chopper noise. There are lower noise, BJT based OPs, but these only work well with low impedance sources (e.g. < 1 Kohms). The OPA827 is even lower noise, but can be tricky with it's high input capacitance.
Having 8 mV pp noise would be about 1.3 mV RMS. With a gain of 100 this corresponds to some 13 µV_rms at the input. This is about the noise level expected for the max4239 (30 nV/Sqrt(Hz) and some 200 kHz BW).
An OP behind the LTC1968 would be not really critical if needed at all. So the LMV321 should be OK.
The LTC1968 might need an OP for driving the input, if there is a high impedance source. DC wise the converter is high impedance, but there is some input capacitance and thus BW would be severely limited with a 2 M source. So for an accurate measurement it needs a buffer/amplifier before the LTC1968.
Besides the price, there are reasons why the better meters usually have a 1 M divider (and not higher impedance) for AC. One point is resistor noise (a 600 K resistor has a white noise level of 100 nV/Sqrt(Hz) and thus about 3 times the max4239. A lower impedance also makes it easier to have at least the 50/60 Hz range divider to be mainly determined by the resistors and thus more accurate.
sourcecharge:
Thanks for the recommendation, I'll check out the opa140/1, and the opa827.
So, ya, I measured a noise of about 8mVpp on the outputs, but then I took the battery out and it was still doing it. It was about 6mVpp but it was about the same garbage.
Then I used the deeper memory function, and I found a 120hz spike of about twice the amplitude of the 8mVpp noise that was varying between 400khz to 4Mhz. I took all of the wires off of the meters while connected to the uCurrent, still doing it. I connect connected the scope probe to it's own ground. Nothing there. I checked the noise from the probes and it was about 2mVpp. There is a 60 hz signal in the air that is about 40 mVpp and even higher when I touch the positive test lead. I'm sure that just residual from the mains but I'm not sure if that is doing something to it. Anyways, I'll do some more test again soon.
I watched some of the eev videos of the uCurrent, and the output of those across dave's scope looks clean and the noise looked like its maybe half of what I was seeing. IDK. I'll try again soon. I might have missed something.
Has anyone noticed that the TPS3809 chip is referenced backwards on the uCurrent?
The function of the chip works, but I measured the resistance between the Vcc and grd pins and they are actually different resistances when reversing the measurement. 160k in one direction and about 98k in the other.
I wonder if that might be causing the some of the problems.
First, I got to find where this noise is coming from even when the battery is taken out.
I'm guessing it might be from my scope.
sourcecharge:
So, I just watched the Murphy video of the uCurrent and it looks like with the ST's LMV321ILT IC, Dave's scope says about 2.5 mVrms, which basically is 8 mVpp so I guess it must be normal. I'm still going to take that voltage supervisor IC off and see if that does anything.
Anyways, so the resistor network seems to be a problem.
I will start by using some normal resistors that are not precision that I have laying around to see if there would be too much noise.
sourcecharge:
What about an inverting amplifier with a gain of 0.001, 0.01, and 0.1?
Then signal then could be routed to a non inverting amplifier with gains of 10, 100, and 1000.
That way the dividing network is limited to only 1 large resistor and 3 switched in resistors for the inverting amplifier that are all tied to the virtual ground when used, and the non inverting amplifier's input signal could go straight to the input leg, thus limiting the capacitance of the divider, and decreasing the resistor noise.
As long as the amplifier has unity gain, and has a high BW, (300Mhz) this thing might actually work up to 150khz.
Does the stability of an opamp change when using it for attenuation?
sourcecharge:
All about inverting amplifiers as attenuators:
http://home.iitb.ac.in/~pradeepsarin/students/tether/analog_gang_2016/TI_e2e_note.pdf
Navigation
[0] Message Index
[#] Next page
[*] Previous page
Go to full version