Someone said this in the YouTube comments too, but so far, Dave has ignored this point.
You are, of course, quite correct.
but most of it flows through the lowest value ... so that portion does get amplified.
No, the "lowest value" has no voltage across it. All of the voltage is across the feedback resistor, and the input bias current flows through it.
Someone said this in the YouTube comments too, but so far, Dave has ignored this point.
You are, of course, quite correct.
Dave makes mistakes like the rest of us (on live video probably less than the rest of us would).
We should point them out but he doesn't have to fess up or apologize or anything. He isn't arguing about it which I just take as acceptance.
but most of it flows through the lowest value ... so that portion does get amplified.
No, the "lowest value" has no voltage across it. All of the voltage is across the feedback resistor, and the input bias current flows through it.
@dtweed
Are you serious or are you mocking the video?
Dave makes mistakes like the rest of us (on live video probably less than the rest of us would).
We should point them out but he doesn't have to fess up or apologize or anything. He isn't arguing about it which I just take as acceptance.
Ordinarily, I'd agree with you. But in this case, Dave makes the error at about 20:00 in the video, and much of what follows is based on this erroneous calculation.
Also, this isn't just some hobbiest putting out random videos, this is now Dave's livelihood, and I would think he'd have a vested interested in getting the fundamentals right.
Also, this isn't just some hobbiest putting out random videos, this is now Dave's livelihood, and I would think he'd have a vested interested in getting the fundamentals right.
I don't think your current measurement with the Keithley 480 was accurate. The input burden voltage of that meter is supposed to be below 200uV. While this figure is very low, it's still way more than those microvolts of error that the bias current caused in the circuit.
but most of it flows through the lowest value ... so that portion does get amplified.
No, the "lowest value" has no voltage across it. All of the voltage is across the feedback resistor, and the input bias current flows through it.
Yes, I certainly formed the view that that was your intention.
Perhaps you can make FF videos available to a selected group of volunteer reviewers (like they do with textbooks) prior to making them available to everyone on YT.
Perhaps I've painted myself into a corner with calling it Fundamentals Friday and people expect it to be somehow different/better fact checked to my regular content?
I never really expected that your video production process would be any different for Fundamental Fridays, but I do have some expectation that you do a little more preparation before starting to shoot, including making a list of the points you intend to cover, and thinking about the sequencing of the presentation.
but most of it flows through the lowest value ... so that portion does get amplified.
No, the "lowest value" has no voltage across it. All of the voltage is across the feedback resistor, and the input bias current flows through it.
Theoretically yes... Measure it and you will see there is a voltage.
I'm just not a huge fan of the Fundamental Fridays series.
i have a noob question
when a opamp datasheet says bias is -10nA
is it trying to say current is going in or out? or it doesnt matter and we can take the value as +/-10nA?
i have a noob question
when a opamp datasheet says bias is -10nA
is it trying to say current is going in or out? or it doesnt matter and we can take the value as +/-10nA?
It means it will source (output) 10nA.
So if you put a 10M resistor across the input to ground you'll measure about 100mV at the input pin.
...
if then assuming @ 187GOhm/square, and we are mapping FR4 with blue squares (see pic). will it be right to assume that pads with long-ish footfalls adjacent to other similar pads suffer a higher leakage as more squares get to fit into the "spaces" ?
ie : 3 squares fitting between 2 pads = 187/3 = 62GOhm (what do we call this? actual creep resistance?)
so with that in mind, would it make sense to use/edit pads that will connect lesser "leaking" squares? i wonder do any PCB designer use this ohm/square method to design high resistance designs?
but strangely, does using a thinner FR4 PCB = higher resistance? ie : 0.8mm board = 375GOhm/square? (i suspect something is wrong with my fundamentals again ha !)