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In my previous question I was thinking about how can I "reuse" this circuit (0-10V, 0-20mA and digital contact input), when I only have 3.3V regulated power available from my LoRa Node:

universal-input

After searching a lot, and without doing some deep changes to the circuitry showed in above picture, I was thinking to use a step-up boost converter using the MCP1640, in order to get 5V output to power the op-amp (LM224D) and leave the rest of the circuit untouched.

This is the recommended design circuit from the datasheet:

stepup-converter

Do you think that this solution will work without any issue? Might it be a good compromise or will it be a total failure?

Edit:

Here you can see the circuit that reference the netflag VLIM:

vlim

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    \$\begingroup\$ If you don't need 300mA, you can use a charge pump. Both are however noisy switching devices so you will end up having a noisy supply unless you take careful measures to filter it enough to be suitable for powering an op-amp. \$\endgroup\$
    – Justme
    Feb 20, 2021 at 9:27
  • \$\begingroup\$ So, what you recommend? I need only to power up the opamp and expose the 5V to header connection for the digital input (marked as DI2) \$\endgroup\$
    – VirtApp
    Feb 20, 2021 at 9:30
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    \$\begingroup\$ What is VLIM in the schematic? Where does it come from? \$\endgroup\$
    – Finbarr
    Feb 20, 2021 at 10:08
  • \$\begingroup\$ @Finbarr, i've edited the post including VLIM section schematic \$\endgroup\$
    – VirtApp
    Feb 20, 2021 at 13:35
  • \$\begingroup\$ @VirtApp Thanks. Just confirm one more thing: you are building a new circuit based on the schematic you posted on the other question (without using the same power circuitry) or you have the device you mentioned and you want to power it with the boost and battery? \$\endgroup\$
    – devnull
    Feb 20, 2021 at 14:12

1 Answer 1

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The VLIM circuit you posted takes the main power input (10.8V to 28.8V) and limits it to around 12.6V (VLIM) which powers the OpAmps.

If the OpAmps you choose can work with single supply @ 5V, rail-to-rail, and the total power consumption is within the specs of your boost converter, I don't see any problems with your new power source.

The LM224D meets the single supply criteria but is not a rail-to-rail device (inputs can't and outputs don't reach Vcc). If you need to use this device for some reason, you'd need to power them with at least 7V and have a separate 5V line for your diode clampers (or lower than that if you need to clamp the outputs to 3.3V).

EDIT:

Regarding the concern about the switching power supply ripple generating noisy analog signals, the boost you selected operates @ 500 kHz, twice the frequency of the buck in the original circuit. If you are also aiming at simple Arduino ADCs, your sampling rate will be much lower than that, requiring anti aliasing filters at least an order of magnitude below the switching frequency. Decoupling capacitors very close to the OpAmps are obviously very important even with linear power supplies (the original circuit also handles digital inputs, which are not predictable and also disturb the power rails).

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  • \$\begingroup\$ So, if i replace the LM224D with the LMV324A, i will solve the issue of rail-to-rail and i'll be able to use the full rail-to-rail 0-3.3V output from op-amp, right? datasheet.lcsc.com/szlcsc/… \$\endgroup\$
    – VirtApp
    Feb 20, 2021 at 14:42
  • \$\begingroup\$ @VirtApp. Yes, but keep in mind that: 1) the output gets very close to but not exactly at Vcc and 0V (dozens of mV in the datasheet I found @ TI); 2) the inputs must be clamped at Vcc and 0V (check the max. ratings at the datasheet). \$\endgroup\$
    – devnull
    Feb 20, 2021 at 14:50
  • \$\begingroup\$ thanks, and using a NCS2006 datasheet.lcsc.com/szlcsc/…, the mV loss can be lower? i want to reproduce the same precision of the original circuit...or maybe a better one \$\endgroup\$
    – VirtApp
    Feb 20, 2021 at 15:02
  • \$\begingroup\$ @VirtApp the typical output swing with the NCS2006 is better by an order of magnitude (both @ 0V and Vcc) but not the worst case. If reaching Vcc is so critical for your application, this one is better than the one in original circuit. \$\endgroup\$
    – devnull
    Feb 20, 2021 at 15:17
  • \$\begingroup\$ I mean is not crytical, but consider this scenario: when the sensor wired to the board outputs 19.7mA or 9.95V...i will loose a lot of precision with the LMV324A instead the NCS2006? \$\endgroup\$
    – VirtApp
    Feb 20, 2021 at 15:22

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