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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?
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.
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.
