The term "reference buffer" is used to describe an amplifier placed between the reference IC and the ADC reference input. The reference buffer responds to very fast transient current requirements on the ADC reference input. Since a capacitor C_bypass is used between VREFHI and VREFLO, the ADC reference can draw current from this capacitor during the conversion of each bit, but the reference circuit needs to recharge the bypass capacitor between bit decisions. If the reference IC alone is unable to fully recharge the capacitor quickly enough, then a reference buffer is needed to preserve ADC performance.

The key specifications for the reference buffer are maximum output current, offset voltage, offset voltage drift, bandwidth, and output impedance across that bandwidth. For definitions of these terms, see Understanding Operational Amplifier Specifications. Low offset voltage and offset voltage drift are needed for DC accuracy across temperature. High bandwidth and low output impedance are needed to respond to fast transient current requirements. For more details, see TI Precision Labs - Driving the reference input on a SAR ADC.