C8051F52X_12 Datasheet, PDF (60/221 Pages) Silicon Laboratories – 8/4/2 kB ISP Flash MCU Family

C8051F52x/F53x
4.3.6. Settling Time Requirements
A minimum tracking time is required before an accurate conversion can be performed. This tracking time is
determined by the AMUX0 resistance, the ADC0 sampling capacitance, any external source resistance,
and the accuracy required for the conversion.
Figure 4.6 shows the equivalent ADC0 input circuit. The required ADC0 settling time for a given settling
accuracy (SA) may be approximated by Equation 4.1. When measuring the Temperature Sensor output,
use the settling time specified in Table 2.3 on page 28. See Table 2.3 on page 28 for ADC0 minimum set-
tling time requirements.
t
=
ln


S^-2---A^{n--}

R_TOTALC_SAMPLE
Equation 4.1. ADC0 Settling Time Requirements
Where:
SA is the settling accuracy, given as a fraction of an LSB (for example, 0.25 to settle within 1/4 LSB)
t is the required settling time in seconds
R_TOTAL is the sum of the AMUX0 resistance and any external source resistance.
n is the ADC resolution in bits (12).
MUX Select
Px.x
R_MUX
C _SAMPLE
R C _Input= R _MUX * C _SAMPLE
Figure 4.6. ADC0 Equivalent Input Circuits
4.4. Selectable Gain
ADC0 on the C8051F52x/52xA/53x/53xA family of devices implements a selectable gain adjustment
option. By writing a value to the gain adjust address range, the user can select gain values between 0 and
1.016.
For example, three analog sources to be measured have full-scale outputs of 5.0 V, 4.0 V, and 3.0 V,
respectively. Each ADC measurement would ideally use the full dynamic range of the ADC with an internal
voltage reference of 1.5 V or 2.2 V (set to 2.2 V for this example). When selecting signal one (5.0 V full-
scale), a gain value of 0.44 (5 V full scale * 0.44 = 2.2 V full scale) provides a full-scale signal of 2.2 V
when the input signal is 5.0 V. Likewise, a gain value of 0.55 (4 V full scale * 0.55 = 2.2 V full scale) for the
second source and 0.73 (3 V full scale * 0.73 = 2.2 V full scale) for the third source provide full-scale ADC0
measurements when the input signal is full-scale.
Additionally, some sensors or other input sources have small part-to-part variations that must be
accounted for to achieve accurate results. In this case, the programmable gain value could be used as a
calibration value to eliminate these part-to-part variations.
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