The I²C bus operates at one of three speeds. Standard mode allows a clock frequency of up to 100 kHz; fast mode permits a clock frequency of up to 400 kHz; and high-speed mode (also called Hs mode) allows a clock frequency of up to 3.4 MHz. The ADS111x-Q1 are fully compatible with all three modes.

No special action is required to use the ADS111x-Q1 in standard or fast mode, but high-speed mode must be activated. To activate high-speed mode, send a special address byte of 00001xxxb following the START condition, where xxx are bits unique to the Hs-capable controller. This byte is called the Hs controller code, and is different from normal address bytes; the eighth bit does not indicate read/write status. The ADS111x-Q1 do not acknowledge this byte; the I²C specification prohibits acknowledgment of the Hs controller code. Upon receiving a controller code, the ADS111x-Q1 switch on Hs mode filters, and communicate at up to 3.4 MHz. The ADS111x-Q1 switch out of Hs mode with the next STOP condition.

For more information on high-speed mode, consult the I²C specification.