uCurrent MOD with inbuilt Display ( CurrentWave D )

[apoorv3in]

Have been working on uCurrent design with
inbuild display, USB Charging, BNC connector for connecting to Oscilloscope.
 CurrentWave D(Display)

[apoorv3in]

[apoorv3in]

Attaching the schematic

[charlesrg]

Great update to the uCurrent, they display is really nice to have.

[mikerj]

The non-precision LMV321 being used as a buffer for the ADC isn't the best if you want accuracy.

[Kleinstein]

With 1:1000 steps for the shunts the likely 12 bit resolution for the µC internal ADC may be a little low, even if one may get some 2 extra bits from oversampling.
I would consider a seprate higher resolution ADC, like MCP3421.

Compared to the µC internal ADC the LM321 may not be that bad, however one may get away with that extra OP. AFAIk the max4238 can also run with 3.3 V.

[apoorv3in]

i am using oversampling with decimation to push to 18bit resolution

[apoorv3in]

Calibration is done in MCU for Slope and Offset. Once calibrated i do not see much deviation.

[SiliconWizard]

Calibration is done in MCU for Gain and Offset. Once calibrated i do not see much deviation.

Yeah, then you make up for the relatively large Vos of the LMV321.
But that means each unit has to be calibrated. Whereas Dave's original design didn't require any calibration AFAIR (but you may reply that the associated multimeter would). Not a big deal, but something to consider if that was meant to be a product manufactured in large quantities.

One thing that we can notice as well, the display is very tiny. So that would be difficult to read for anyone with challenged eyesight, or if the unit is not right next to you.

[apoorv3in]

This device will not be mass produced. I assemble/put each component by hand so calibration is least of my worry. I can use the 2 buttons on unit to calibrate as I already have the raw output. Regarding the screen size, I gave it lot of thought when doing the design I tried all screen sizes but this was the only one I could fit. With my manufacturing limitation and trying to keep a clean look (single sided layout) I had to make compromise on screen size.

[Kleinstein]

The offset of the LMV321 OP would effect the overall offset. The adjustment / calibration for this would be a simple zero test - so it could be easily repeated and does not need extra equipment.

With an offset adjustment there would be a larger number of AZ OPs to choose from, as the offset would not be a critical parameter anymore.
The max4239 is nice with a low offsert and low bias, but also relatively high noise.