22.3.6.2 Advanced Full-Scale Voltage Configuration
For most applications, the pre-defined VFS options described in
22.3.6.1 Basic Full-Scale Voltage Configuration
are suitable. Advanced
VFS configurations are also possible by programming the REF bitfield in ADCn_SINGLECTRL or ADCn_SCANCTRL to the CONF op-
tion. Programming the REF bitfield to CONF allows the user to select the specific VREF source and adjust the programmable input and
reference divider options directly.
The general procedure for programming an advanced VFS configuration is as follows:
1. Select the voltage reference source using VREFSEL.
2. Configure VREFATTFIX and VREFATT so that the reference voltage at the ADC is between 0.7 and 1.05 V.
3. Configure VINATT to achieve the desired full-scale voltage.
The VREFSEL field in ADCn_SINGLECTRLX or ADCn_SCANCTRLX selects the voltage reference source. The ADC can choose from
the following voltage reference (VREF) sources:
• VBGR: An internal 0.83 V bandgap reference voltage. This is the most precise internal reference source available.
• VDDXWATT: An attenuated version of the AVDD supply voltage. The attenuation factor is determined by the VREFATTFIX and/or
VREFATT bit fields.
• VREFPWATT: An external reference source applied to the ADCn_EXTP pin, and attenuated by the attenuation factor (determined by
the VREFATTFIX and/or VREFATT bit fields). This is the appropriate choice for external reference inputs greater than 1.05 V.
• VREFP: An external reference source applied to the ADCn_EXTP pin, without any attenuation. This is the appropriate choice for
external reference inputs between 0.7 V and 1.05 V.
• VENTROPY: A very low internal reference voltage (approx. 0.1 V). This option is intended to be used only with the ADC inputs tied
internally to VSS, for generating random noise at the ADC output.
• VREFPNWATT: A differential version of VREFPWATT, with the reference source applied to the ADCn_EXTP and ADCn_EXTN pins
and attenuated. This is the appropriate choice where a differential reference of greater than 1.05 V is required.
• VREFPN: A differential version of VREFP, with the reference source applied to the ADCn_EXTP and ADCn_EXTN pins and no at-
tenuation. This is the appropriate choice where a differential reference of between 0.7 V and 1.05 V is required.
• VBGRLOW: An internal 0.78 V bandgap reference voltage.
The ADC reference voltage should be attenuated to a lower voltage when using AVDD or the external reference source. A simple meth-
od for a wide range of reference sources is to set VREFATTFIX to 1. The VREF attenuation factor (ATT
VREF
) can then be selected
between 1/3 (when VREFATT is greater than 0), and 1/4 (when VREFATT is equal to 0). For reference sources between 1.2 V and 3.6
V, ATT
VREF
= 1/3 is the best choice. ATT
VREF
= 1/4 can be used with references from 1.6 V to 3.8 V, with slight performance degrada-
tion.
Finer granularity on ATT
VREF
is possible as well, by clearing VREFATTFIX to 0, and setting the VREFATT field. For optimal perform-
ance with VREFATTFIX = 0, the attenuated ADC reference input should be limited to between 0.7 V and 1.05 V. When VREFATTFIX is
cleared to 0, ATT
VREF
is set according to the equation:
ATT
VREF
= (V 6) / 24 for VREFATT < 13, and (VREFATT - 3) / 12 for VREFATT ≥ 13
Figure 22.9. ATT
VREF
: VREF Attenuation Factor
The ADC input also includes a programmable attenuator. The VIN attenuator is used to widen the available input range of the ADC
beyond the reference source. The VIN attenuation factor (ATT
VIN
) is determined by the VINATT field according to the equation:
ATT
VIN
= VINATT / 12 for VINATT ≥ 3 (settings 0, 1, and 2 are not allowable values for VINATT)
Figure 22.10. ATT
VIN
: VIN Attenuation Factor
VFS can be calculated by the formula given below for any given VREF source, VREF attenuation, and VIN attenuation:
VFS = 2 × VREF × ATT
VREF
/ ATT
VIN
VREF is selected in the VREFSEL bitfield, and
ATT
VREF
is the VREF attenuation factor, determined by VREFATT or VREFATTFIX
ATT
VIN
is the VIN attenuation factor, determined by VINATT
Figure 22.11. VFS: Full-Scale Input Range
EFM32JG1 Reference Manual
ADC - Analog to Digital Converter
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