8.2.2.1 Estimating ADC Reporting Accuracy

The UCD90120A uses a 12-bit ADC and an internal 2.5-V reference (V_REF) to convert MON pin inputs into digitally reported voltages. The least significant bit (LSB) value is V_LSB = V_REF/2^N where N = 12, resulting in a VLSB = 610 μV. The error in the reported voltage is a function of the ADC linearity errors and any variations in VREF. The total unadjusted error (E_TUE) for the UCD90120A ADC is ±5 LSB, and the variation of VREF is ±0.5% between 0°C and 125°C and ±1% between –40°C and 125°C. V_TUE is calculated as V_LSB × E_TUE. The total reported voltage error is the sum of the reference-voltage error and V_TUE. At lower monitored voltages, V_TUE dominates reported error, wheereas at higher monitored voltages, the tolerance of V_REF dominates the reported error. Reported error can be calculated using Equation 3, where REFTOL is the tolerance of V_REF, V_ACT is the actual voltage being monitored at the MON pin, and V_REF is the nominal voltage of the ADC reference.

Equation 3.

From Equation 3, for temperatures between 0°C and 125°C, if V_ACT = 0.5 V, then RPT_ERR = 1.11%. If V_ACT = 2.2 V, then RPT_ERR = 0.64%. For the full operating temperature range of –40°C to 125°C, if VACT = 0.5 V, then RPT_ERR = 1.62%. If V_ACT = 2.2 V, then RPT_ERR = 1.14%.

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