Micropython support LTC1864

Discuss development of drivers for external hardware and components, such as LCD screens, sensors, motor drivers, etc.
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rkompass
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Joined: Fri Sep 17, 2021 8:25 pm

Re: Micropython support LTC1864

Post by rkompass » Wed Nov 24, 2021 9:51 pm

In the original schematics IN+ is shifted to 2.5 V i.e. should output about 32767, as you observed.
The wiring of the input divider resistors is so, that the voltage at Vin is multiplied by a ratio of 38.1k/2038.1k i.e. it is reduced to about 1.8% of the original value. So if your input voltage varies by about +/-4V it would yield 75 mV variation at IN+ resulting in +/-980 measurement value.
This fits nicely to what you abserved.
So the voltage divider resistors seem to be the problem.

rkompass
Posts: 9
Joined: Fri Sep 17, 2021 8:25 pm

Re: Micropython support LTC1864

Post by rkompass » Wed Nov 24, 2021 10:08 pm

I agree with Robert to remove R5 + R6. They could possibly be replaced by other limiting diodes of the TVS type which clamp the input to about 2.5 Volts. R3+R4 are still much to high. In the datasheet an input leakage current of 1 uA is mentioned and an OPA for bringing the input source impedance to below 200 Ohms recommendend. Perhaps a R3+R4 value of 100-330 Ohm is a good match.

nir_bec
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Joined: Mon Aug 09, 2021 10:38 am

Re: Micropython support LTC1864

Post by nir_bec » Thu Nov 25, 2021 6:59 am

I'm working with gained photodiode that outputs 0-5V, so clamping to 2.5V will be a problem for me.
Is this something achievable using the current ADC11 Click schematic? or should i look for another LTC1864 module?

Thank you for the support, :!:

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Roberthh
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Location: Rhineland, Europe

Re: Micropython support LTC1864

Post by Roberthh » Thu Nov 25, 2021 7:16 am

You could remove R7 and C3 and make the connection between IN- and GND of the module. Then you have the full 0-5 V range. If you do not need fast sampling, you can keep C3.

You get similar results by:
- removing R7 and R6,
- replace R3 and R4 by a wire or resistors with a smaller value, like R6 and R5.
- Connect IN- with the board GND.
Then you still have IN- and IN+ for connection, and a range of 0-5V.
In any case you have to run a calibration to match the values you read with the real voltage and eventually the light intensity.

nir_bec
Posts: 12
Joined: Mon Aug 09, 2021 10:38 am

Re: Micropython support LTC1864

Post by nir_bec » Sun Nov 28, 2021 8:37 am

Hi,
After reworking the PCB as you suggested its works like a charm, both code provided and 0-5V range.

Thank you!

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