According to datasheet of MCP3021 A/D converter (Microchip, page 16) the MCP3021 can be accessed by proper address bits in control byte. In case of MCP3021 5AT the address is 101, in case of MCP3021 4AT the address is 100. Does than mean that to interface two MCP3021 diveces in one circuit one have to purchase two devices with different serial number, e.g. MCP3021 5AT and 4AT?
Yes, you need different devices for different addresses. Up to 8 addresses are possible:
The first part of the control byte consists of a 4-bit device code, which is set to 1001 for the MCP3021. The device code is fol- lowed by three address bits: A2, A1 and A0. The default address bits are 101 (contact the Microchip factory for additional address bit options).The address bits allow up to eight MCP3021 devices on the same bus and are used to determine which device is accessed
The number after the A in the part number indicates the address:
Off the shelf you get address 5. If you want other addresses you have to order them specially from Microchip.
The device sacrifices flexibility in favour of size. If you want multiple devices on the same bus and not have to order different parts specially then you generally end up with a bigger chip with more pins to allow you to manually configure the sub-address.