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11 x Analog Sensors to ADC ?
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11 x Analog Sensors to ADC ?

by jonamleonel on Mon Aug 10, 2020 6:27 pm

Hello,

Any recommendation for connecting 11 analog sensors(0-3.3V) to a Particle Boron (it only has 5 analog inputs)?

I will be connecting the 11 analog sensors to a Particle Boron with this IoT Node
https://sentientthings.com/products/iot-node

jonamleonel
 
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Re: 11 x Analog Sensors to ADC ?

by adafruit_support_mike on Tue Aug 11, 2020 8:48 pm

The simplest option would be to use external ADCs, like the ADS1015 or ADS1115:

https://www.adafruit.com/product/1083
https://www.adafruit.com/product/1085

Those communicate using I2C, so multiple sensors can all use the same pair of SDA/SCL pins. With two external ADCs and the five built into the Boron, you could handle a total of 13 analog signals.

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Re: 11 x Analog Sensors to ADC ?

by jonamleonel on Tue Aug 11, 2020 9:44 pm

@adafruit_support_mike Does ADS1115 have pullup resistors included? What happen if the master (IoT node)has already pullup resistors?

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Re: 11 x Analog Sensors to ADC ?

by adafruit_support_mike on Wed Aug 12, 2020 12:04 am

The breakout does have 10k pull-up resistors on the SDA and SCL lines, and their effect on an I2C bus that already has pull-ups will be negligible.

Theoretically, I2C pull-ups should be scaled so it takes about 1mA to pull them low.. 4.7k for 5V circuits or 3.3k for 3.3V circuits. In practice, the average microcontroller pin can sink at least 5mA, and would work with any value down to 1k @ 5V or 680 @ 3.3V.

The 10k pull-ups on a pair of ADS1115 breakouts would combine to act as a 5k pull-up (the theoretical value for a 5V bus). If they were connected to a bus that already has 4.7k pull-ups, it would increase the pull-down current to about 2mA. The rising edges of the waveforms would be a little faster, but it wouldn't have any noticeable effect on communication or operating the microcontroller.

You'd need to put about 15 boards in parallel to push the limits of a 3.3V microcontroller. At that point (actually above about 5 boards), it's a good idea to start removing some of the pull-ups.

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Re: 11 x Analog Sensors to ADC ?

by jonamleonel on Fri Aug 14, 2020 5:00 am

Nice work. Thanks a lot. I will try that!

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Please be positive and constructive with your questions and comments.