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The MCP3021 is a standard 10-bit ADC with an I2C Interface. It's datasheet can be found here.

In the general description, it says:

The device is also addressable, allowing up to eight devices on a single 2-wire bus.

What does it mean to be addressable, how are devices addressed, and why does it say only 8 devices are allowed on one bus? Shouldn't it be much more?

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    \$\begingroup\$ Answer on pg22 8 addresses available \$\endgroup\$ – sstobbe May 22 '17 at 3:26
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In case of MCP3021, the value of the I2C address is "hard-wired" inside of the IC. Internally hard-wired address allows a package with a smaller number of pins, but it takes away the ability to choose the address by pin strapping.

Exposed I2C address pins - and an address configurable by pin strapping - is the opposite of a hard-wired address. Exposed address pins, are a norm rather than exception. There are plenty of A/D chips that allow you to select the I2C address by pin strapping. Four (4) bits is the largest number of selectable address bits that I've seen so far was.

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Unlike most i2c devices, this one has no user adjustable address pins. The 3 address bits are set internally at the factory. You must buy different part numbers so you can put more than one on a single i2c bus.

What it means by addressable is that i2c requires a unique device address for communication on the bus. It's part of its protocol. Since this device only has 3 bits of addressing, only 8 unique addresses can be on a bus. You could use more by having multiple buses or a bus switch or multiplexer.

Or just get a device with multiple adc inputs.

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