Internal Reference Voltage Droop under External Analog Loading
External analog load currents extract direct current and charge pulses from internal bandgap reference nodes, inducing static droop and transient conversion errors.
SAR ADC
Conversion Circuit
Analog-to-digital conversion using a binary search algorithm provides a balanced trade-off between resolution and sampling speed. An architecture known as a sar adc utilizes a successive approximation register to determine each bit of the digital output sequentially. This design requires only a single comparator, which minimizes power consumption.
Sampling Phase
Input signal acquisition occurs during the tracking phase when the internal sampling capacitor is connected to the input source. For a sar adc, the sampling capacitor must be fully charged to the input voltage before the conversion phase begins. Any source impedance limits the current flow, increasing the required acquisition time and restricting the maximum sampling rate.
This limitation is typically addressed by placing a low-impedance driver amplifier before the converter.
DAC Settling
Internal digital-to-analog converter settling time determines the maximum speed of each conversion step. During the search process, the sar adc updates the internal digital-to-analog converter, which must stabilize to within a fraction of a least significant bit before the comparator makes its decision. Insufficient settling time introduces non-linearity and missing codes into the digital output.
Dynamic errors of this type are most pronounced when converting high-frequency signals.
Input Drive
Amplifier selection is critical to ensuring the input signal remains stable during the rapid charge transfer of the sampling phase. The driver amplifier must have a high bandwidth and low output impedance to recharge the sampling capacitor quickly without oscillating. If the amplifier takes too long to recover from the charge injection, the sampled voltage will be incorrect.
This recovery behavior is especially critical in multiplexed systems where the input voltage can change abruptly between channels, requiring rapid settling to maintain channel-to-channel isolation. Designing the interface with a low-pass filter between the amplifier and the converter helps absorb these transient current spikes and stabilizes the conversion.