Products > Test Equipment
Cheap Chinese Oscilloscope Preamp Tear Down/Review – Is It Any Good?
switchabl:
There are not actually that many fast inamps on the market. The AD8421 would seem to fit the bill (https://www.analog.com/media/en/technical-documentation/data-sheets/ad8421.pdf). If you take a look at fig. 52-56 in the datasheet, you see that it basically works as expected. It isn't really compensated for G=1 (or G=10).
The unterminated BNC cable essentially looks like a 100pF/m capacitive load at low frequencies, so the phase margin is reduced further. A 50 Ohm series resistor at the output would likely help with that. It doesn't solve the core problem though.
But to be honest: if you enjoy tinkering with this thing that is hard to argue with. If the goal is to have a working preamp, I seriously believe that it would be easier to start from scratch.
rvalente:
After a few more months of use, have you experienced any issues?
gby:
I would say the only thing to add to the original review is that the device self discharges the battery when just sitting there and off after a couple of weeks. So, basically any time you want to use it you have to charge it up for an hour or two first.
I haven't used it much due to the fatal flaw of the switching supply noise coupling to the output.
BennoG:
for those who want to reverse engineer here are some x-ray images of the board.
they are where 16bit tiff files converted to 32bit png files.
I am also drawing it in kicad but have little time for it.
Benno
BennoG:
As promised the kicad files including pcb layout.
ToDo: values of resistors and capacitors.
ToDo: multiplier resistors for the opamp.
The main problem is the output of the amplifier runs under 1 of the coils of the switching power supply.
I have cut the traces and short them to the ground plane. And use a thin coaxial cable from the output of the amplifier under the accu and then to the output.
This decreases the switching noise a lot.
Next step is to decouple the power supply from the LDO's in some way.
Benno
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