Products > Test Equipment

PRP1 - Low cost 2GHz power rail probe

<< < (4/13) > >>

nctnico:

--- Quote from: tszaboo on August 23, 2024, 05:45:05 pm ---
--- Quote from: nctnico on August 23, 2024, 05:13:48 pm ---There are up/down button potmeter chips which don't require any programming / software. Just hook up some buttons to pins and off you go.

--- End quote ---
I made a quick search and for example the MAX5450 would be suitable. It's 256 tap, that would bring the resolution of nulling to 180mV. So at least two would be needed.
So browsing through the datasheets of these, you would need to press the up/down button 256 times to swipe across the range. I don't think it would be a good user experience.
I also recall using some AD digipots, and they had quite bad tempco. These seem fine though from that point of view.

--- End quote ---
You still have the offset control of the DSO itself so 256 taps would be overkill. 64 would be more than enough. Also, do you really need a 24V range? It would make more sense to have a 4V and 24V range which then translates into a 64 tap digital potmeter having plenty of resolution either way.

temperance:

--- Quote ---At least my ten turns trimmer pot was definitely making more noise than these Alps potentiometers.
--- End quote ---

I don't know what your schematic looks like but I assume you are just injecting an off-set into some amplifier input. Adding a filter might work. (but not with X7R capacitors) The circuit cellar schematic has a mistake in the schematic. The filter cut-off varies with the pot setting from no filter to some filter when the pot is adjusted to the center position.

I prefer pot's over push buttons. Faster and less fiddling.

Is there a reason why the front and back are made of aluminum? You could use FR4 and no extra manufacturing steps would be required.

Edit: the Circuit cellar schematic might have more problems. It is meant to be powered from alkaline AA cells. Some battery chemistries show some ultra low frequency noise where the voltage changes all of a sudden 50...150 mV or even more for some lithium batteries. Those transient lasts from several seconds up to hours.

nctnico:

--- Quote from: temperance on August 23, 2024, 08:54:27 pm ---
--- Quote ---At least my ten turns trimmer pot was definitely making more noise than these Alps potentiometers.
--- End quote ---

I don't know what your schematic looks like but I assume you are just injecting an off-set into some amplifier input. Adding a filter might work. (but not with X7R capacitors) The circuit cellar schematic has a mistake in the schematic. The filter cut-off varies with the pot setting from no filter to some filter when the pot is adjusted to the center position.

I prefer pot's over push buttons. Faster and less fiddling.

--- End quote ---
I get that, but long term potmeters tend to wear. Back when I was designing audio gear, I always filtered a potmeter to provide a DC level to a chip which did the actual control work (like changing the volume). In this case a potmeter would need a filter + buffer in order to get rid of nasty potmeter effects. But you can still get a slow wandering DC offset.

tszaboo:

--- Quote from: temperance on August 23, 2024, 08:54:27 pm ---
--- Quote ---At least my ten turns trimmer pot was definitely making more noise than these Alps potentiometers.
--- End quote ---

I don't know what your schematic looks like but I assume you are just injecting an off-set into some amplifier input. Adding a filter might work. (but not with X7R capacitors) The circuit cellar schematic has a mistake in the schematic. The filter cut-off varies with the pot setting from no filter to some filter when the pot is adjusted to the center position.

I prefer pot's over push buttons. Faster and less fiddling.

Is there a reason why the front and back are made of aluminum? You could use FR4 and no extra manufacturing steps would be required.

--- End quote ---
I have to agree, pot is also my preference. There are a few issues with that circuit, that make it less than optimal. For example your offset voltage constantly changes as your battery discharges.
I was using a 10 turns trimpot to bring up the circuit and test it. They were creating strange low frequency drift. I have to assume, that pushing the plastic housing of the trimpot introduces stress that will upset uV level signals. The time constant was seconds, no amount of capacitors will filter that. With these potentiometers it works much better.

tszaboo:

--- Quote from: nctnico on August 23, 2024, 06:17:43 pm ---You still have the offset control of the DSO itself so 256 taps would be overkill. 64 would be more than enough. Also, do you really need a 24V range? It would make more sense to have a 4V and 24V range which then translates into a 64 tap digital potmeter having plenty of resolution either way.

--- End quote ---
With +/- 24V that the probe can offset out, and +/-500mV,  you can only ever investigate 1V around the setpoint with the probe. You need to bring the output signal of the PRP1 close to 0 otherwise any ripple could be clipped. With 48V/64 tap, the resolution is only 0.75V.
It's also worth noting that while you sweep the offset voltage range, nothing happens on the oscilloscope. If your potentiometer is set to 0V compensation, and the signal is 20V, the pot needs to be turned all the way to 19.5V before the signal moves. You can do this with a fraction of a second with a pot, but with a digipot, it's pressing buttons and nothing happens. Not a very nice user experience.

Navigation

[0] Message Index

[#] Next page

[*] Previous page

There was an error while thanking
Thanking...
Go to full version
Powered by SMFPacks Advanced Attachments Uploader Mod