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All Springer/NP/PCP Air Gun Discussion General > Engineering- Research & Development

Trying to add pressure sensors to LDC testing.

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WhatUPSbox?:
Bruce, Here is a schematic of my current (problematic) set-up. If I test with just one sensor and trigger off of its signal rather than the laser, I think I'm OK. It is just a differential measurement across the bridge. Once I add the second sensor and/or the laser module, the common in the DSO ties it to the bridge and the output is affected.

I am looking at instrumentation amplifiers. Either the Analog Devices AD62X series or the AD8221. That will turn the bridge outputs into single ended inputs into the DSO.
Of course the right answer is to use the Endevco 136 amplifier but even surplus they are out of hobby budgets.

I'm not quite sure if and where I should attach the cable shield that is in the Endevco cables. 

WobblyHand:
Stan, yes, not a good idea to common those bridge circuits.  An instrumentation amplifier would be best for this, although I'd think using an op amp as a differential amplifier would work as well.  But to make it work well, you'd need to match the resistors carefully.  Even then, an instrumentation amp would be much better.  How much gain do you need?  What kind of bandwidth?  1 MHz ok?

Here's https://www.analog.com/media/en/technical-documentation/data-sheets/LT6370.pdf which might be suitable if you can get it in a decent package.  Made for bridge applications, I think.  Maybe $6 each, I'd guess.  But something like this might work.

This might be better.  https://www.analog.com/media/en/technical-documentation/data-sheets/ad8422.pdf.

For all of them, check the gain bandwidth product to see if you can get both the gain and the bandwidth you need.  If GBW=1MHz, and you ask for a gain=2, you will only get 500KHz of bandwidth.  So for BW=1MHz and a desired gain of 10, the GBW has to be greater than 10MHz.

WhatUPSbox?:
Bruce, thank you for the recommendations. I'll have to decide on packaging. The AD620 and AD626 I can find in DIP. I do have a few sop8 breakout boards so the AD8422 is an option.

WobblyHand:

--- Quote from: WhatUPSbox? on March 05, 2024, 10:23:19 PM ---Bruce, thank you for the recommendations. I'll have to decide on packaging. The AD620 and AD626 I can find in DIP. I do have a few sop8 breakout boards so the AD8422 is an option.

--- End quote ---
I'm finding that getting DIP packaging for any modern parts is impossible.  I've even had to resort to using DFN packages which have no leads and the pads are on the bottom of the part.  For prototyping I had to buy assortments of breakout boards, for DFN-8, SOT-6, SOP8, SSOP8, etc.  I simply couldn't procure the parts in the larger packages, they are no longer being made.  I use a hot air gun to solder the SMT parts to the boards.  Definitely harder than it used to be - in the old days.  Now I have to use a microscope to solder parts.

WhatUPSbox?:
Bruce, any thoughts on the AD8226 compared to the AD8422? Slightly slower bandwidth but the lower cost makes for extra soldering spares ;)

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