High Speed Analog Design and Application Seminar
High-Speed ADC References
ɷ Typically designed to meet ac specs
ɷ Not a ‘precision’, low-drift Reference
ɷ Limited drive capability
ɷ Inherently good matching for multi-channel ADCs, e.g. octal
ɷ May be used to improve gain matching between devices
ɷ Typically complex circuit
• Needs high-speed properties
• Drive capability and Cload stability
• May not be tracking; Range, drifts
Often the specification around the internal references are scarce. In some cases
the reference performance is included in the overall ADC specifications such as
gain error, or gain error drift.
In other instances errors caused by the references are separated from the rest of
the converter. For example, the ADC may have a gain error spec which is based
on operating it in the external reference mode. In order to understand the total
gain error one would have to combine the ADC error and the reference error.
In most cases the ADC and its reference form an entity designed primarily to
achieve the ac-performance goals. DC precision is usually compromised. This
might be a limitation especially for dc coupled time domain applications. The drift
performance of the internal reference is typically moderate with around 20ppm/°C.
Also, the internal references are designed to accommodate the demands of the
ADC core. There is usually very little drive capability left to supply any external
circuitry. In most cases external buffer are required.