Electrical – I2C ADC ADS7830 and Digital Isolator MAX14850 – reading central heating system thermistors


I want to read the temps from my central heating system.
I have the ADC ADS7830 and the digital isolator MAX14850.

I need help with the circuit.
This are the values of the different thermistors (not all because i can't post more then 2 links):

Thermistor values

I have the Adafruit LM4040 voltage reference here but it has only 4.096V.

The ADS7830 datasheet says on page 11:

If a +5V supply is used, an external +5V reference is required in
order to provide full dynamic range for a 0V to +Vdd analog input.

What if i need to read 6V? I guess i need a voltage divider?

Then page 18 the layout:

The GND pin should be connected to a clean ground point. In many
cases, this will be the "analog" ground. Avoid connections that are
too near the grounding point of a microcontroller or digital signal
processor. The ideal layout will include an analog ground plane
dedicated to the converter and associated analog circuitry.

They talk about GND and not COM. Not connecting GND to the supply and voltage reference? Do i link GND and COM?

What about the power for the voltage reference? I already have a dedicated power supply for the relay board to have real isolation. But i guess using it also to power the voltage reference is not a good idea because the relays can cause noise and voltage fluctuation?

First draft (not sure about COM):
enter image description here

Best Answer

A few pointers: -

  • The ADC has an internal reference voltage that is quite suitable for reasonably accurate measurements so please consider ditching the external reference unless you have some cunning plan in mind that is not apparent from your question.
  • Thermistors are just resistors so you need a pull-up resistor to the local supply in order to generate a signal voltage that can be measured by the ADC.
  • If you buffer the reference voltage output you can use this as your pull-up point for the pull-up resistors then you gain accuracy in that the measurements are ratiometric.

They talk about GND and not COM. Not connecting GND to the supply and voltage reference? Do i link GND and COM?

The common pin has to be within +/- 0.2 volts of the ground pin - see "Absolute Input Range" on pin 3 and note that "Negative Input" refers to the common pin.

You do need an isolated power supply for the isolated side of the circuit - consider using an isolating dc-dc converter or find an isolator that generates isolated power. ADI's ADuM540x range does this and also provide the isolated IO for the device: -

enter image description here

Be aware that the isolated power than can be provided is limited so, whatever you choose for pull-up resistors be economical in taking current.

My biggest worry is the commoning up of all the thermistors located around your house to the common point on the ADC. There could be a lot of induced voltage due to these components sharing pathways with regular AC power signals. It's something you need to consider and consider it strongly.