![](http://datasheet.mmic.net.cn/Energy-Micro/EFM32TG840F16_datasheet_97240/EFM32TG840F16_432.png)
Preliminary
...the world's most energy friendly microcontrollers
2011-05-19 - d0034_Rev0.91
432
www.energymicro.com
the OPA output by setting corresponding bits in OUTPEN in DACn_OPAxMUX. For OPA0 alternative
output 4 is connected to ADC input mux CH0 when enabled. OPA1's alternative output 4 is connected
to ADC input mux CH1 when enabled. For OPA2, the two main outputs can be connected to ADC input
mux CH0 and ADC input mux CH5 respectively when enabled. In addition to these, OPA0 and OPA1
the ADC chapter for information on how to configure the ADC input mux.
26.3.1.3 Gain Programming
The feedback path of each mux includes a resistor ladder, which can be used to select a set of gain
values. The gain can be selected by the RESSEL bitfield located in DACn_OPAxMUX register. The
gain values are taken from tappings of the resistor ladder based on ratio of R2/R1. It is also possible to
bypass the resistor ladder in Unity Gain (UG) mode.
26.3.1.4 Offset Calibration
The offset calibration registers are located in different registers for the opamps. OPA0 and OPA1's offset
can be set through the CH0OFFSET and CH1OFFSET bitfields respectively in DACn_CAL. The offset
for OPA2 can be set through OPA2OFFSET in DACn_OPAOFFSET.
26.3.1.5 Shorting Non-inverting and Inverting Input
Functionality for offset calibration of the opamps has been added, this functionality is enabled by setting
the OPAxSHORT bitfield in DACn_OPAxCTRL. Setting this bitfield enables a switch that shorts between
the inverting and non-inverting input of the OPA, effectively driving the offset voltage of the opamp to
the output. Using the ADC to measure this offset, the calibration register can be adjusted to minimize
the output offset.
26.3.1.6 Low Pass Filter
The low pass filter is located between the pad and the positive input. The lowpass filter is designed to
couple the input signal to local VSS for high frequencies and has a 3dB frequency of approximately
130MHz when driven from a 50 ohm source. The filter adds a parasitic capacitance of approximately
1.2pF towards local VSS when enabled. The filter is enabled out of reset and can be disabled by setting
OPAxLPFDIS in DACn_OPAxCTRL.
26.3.1.7 Disabling of rail-to-rail Operation
Each opamp can have the input rail-to-rail stage disabled by setting the OPAxHCMDIS bitfield in
DACn_OPACTRL. Disabling the rail-to-rail input stage improves linearity of the opamp, thus improving
the Total Harmonic Distortion, THD, at the cost of reduced input signal swing.
26.3.2 Opamp Modes
The opamp can be configured to perform different Operational Amplifier functions by configuring the
internal signal routing between the opamps. The modes available are described in the following sections.
26.3.2.1 General Opamp Mode
In this mode the resistor ladder is isolated from the feedback path and input signal routing is defined
by OPAxPOSSEL and OPAxNEGSEL in DACn_OPAxMUX. The output signal routing is defined by
OUTPEN in DACn_OPAxMUX
Table 26.1. General Opamp Mode Configuration
OPA bitfields
OPA Configuration
OPAx POSSEL
POSPADx, DACx