SBASAM0A March 2024 – June 2024 ADS127L18
ADVANCE INFORMATION
A reference voltage is required for operation. The reference voltage input is differential, defined as: VREF = VREFP – VREFN, and is applied to the REFP and REFN inputs. All ADC channels use the same reference voltage. See the Reference Voltage Range section for details of the reference voltage operating range.
As shown in Figure 7-5, the reference input sampling structure is similar to the analog input structure. ESD diodes protect the reference inputs and turn on when the reference pin voltage thresholds are exceeded. To keep these diodes off, make sure the reference pin voltages do not go below AVSS by more than 0.3V or above AVDD1 by 0.3V. If these conditions are possible, use external clamp diodes, series resistors, or both to limit the input current to the specified value.
The reference voltage is sampled by a sampling capacitor CREF. In unbuffered mode, current flows through the reference inputs to charge the sampling capacitor. The current consists of a dc component and an ac component that varies with the frequency of the modulator sampling clock. See the Electrical Characteristics table for the reference input current specification.
Charging the reference sampling capacitor requires the external reference driver to settle at the end of the sample phase t = 1 / (2 · fMOD). Incomplete settling of the reference voltage increases gain error and gain error drift. Operation in the lower speed mode reduces the modulator sampling clock frequency, therefore allowing more time for the reference driver to settle.
A precharge buffer option is available for the REFP input to reduce the charge drawn by the sampling capacitor. The precharge buffer provides the coarse charge for the reference sampling capacitor CREF. Halfway through the sample phase, the precharge buffer is bypassed (S1 is in an up position as demonstrated in Figure 7-5). At this time, the external driver provides the fine charge to the sampling capacitor. Because the buffer reduces the charge demand of the sampling capacitor, the bandwidth requirement of the external driver is greatly reduced. The sampling current through the REFN input is not reduced by the REFP buffer.
Many applications either ground REFN, or connect REFN to AVSS. A precharge buffer for REFN is not necessary for these cases. For applications when REFN is not a low-impedance source, consider buffering the REFN input.