OA Operation
861
SLAU208Q – June 2008 – Revised March 2018
Copyright © 2008–2018, Texas Instruments Incorporated
Operational Amplifier (OA)
33.2 OA Operation
The OA module is configured with user software. The setup and operation of the OA is discussed in the
following sections.
33.2.1 OA Modes
The OA can be enabled and disabled using OAM as listed in
. To speed up the enable time of
the OA, The REF module REFON bit in the REFCTL register can be set before enabling OA to allow the
AFE bias, required by the OA, to turn on.
Table 33-1. OA Mode Select
OAM
OA Function
0
Disabled. Output high impedance. All input switches opened, regardless of PSW or NSW settings.
1
Enabled. If OA output is shared with a digital I/O port, the digital I/O port is automatically disabled regardless of the
PxSEL settings of the respective port.
33.2.2 Rail-to-Rail Input Modes
Each operational amplifier supports rail-to-rail input swings. Setting OARRI = 1 enables the rail-to-rail input
mode but requires additional current. Thus, to save current, rail-to-rail input mode (OARRI) should be
selected only if the dynamic range required for the application requires it. Refer to the common-mode
voltage range V
CM
specified in the device-specific data sheet for OARRI = 0 and 1. Rail-to-rail operation is
made possible by an integrated charge pump that increases the internal supply of the respective
amplifier's input stage to allow linear operation over the complete common mode range. The charge pump
is shared with all operational amplifier instantiations. Setting any OARRI = 1 of any respective OA will
enable the charge pump independently of the OAM setting. The charge pump can also be enabled from
the CTSD16 module if in rail-to-rail input mode, CTSD16RRI = 1. When OARRI = 1 is set, the charge
pump, if not already enabled, is enabled. OARRIRDY = 1 indicates the charge pump is ready and stable,
and the rail-to-rail input mode is ready for usage. Setting OARRI = 0 causes OARRIDY = 0 for the
corresponding OA. The charge pump does incur additional power to operate. The charge pump requires a
single external capacitor from the CPCAP terminal to ground for proper operation. See the device-specific
data sheet for proper value and tolerances. Setting OARRI = 0 provides for a reduced input swing range
on the respective amplifier. Setting OARRI = 0 of all OA instances and setting CTSD16RRI=0 is required
to disable the charge pump. See the device-specific data sheet for input ranges supported for all OARRI
settings.
NOTE:
On devices that contain the CTSD16, the charge pump is also used for PGA buffers when
rail-to-rail input mode is selected, CTSD16RRI = 1. Therefore, the charge pump may remain
enabled although OARRI = 0 for all OA instances.
NOTE:
On devices that contain the CTSD16, the charge pump uses a clock that is provided by the
CTSD16CLK. If any OA is enabled and set to rail-to-rail input mode, the CTSD16CLK will
automatically be enabled regardless if the CTSD16 is enabled or not. In addition, the
CTSD16CLK has fault detection logic that may cause an NMI to occur. Any existing fault
should be corrected for proper OA operation.
33.2.3 OA Inputs
The OA has a configurable input selection. The signals for the + and – inputs are individually selected with
the OAxPSW and OAxNSW bits, respectively. Each switch is independently selectable. The connection of
inputs can either be internal or external connections and are device specific. Refer to the device-specific
data sheet for all operational amplifier connections. When OAM = 0, the OA is disabled and all input
switches are opened regardless of the OAxPSW and OAxNSW settings.
Negative input terminal switch 4 control, NSW4, allows for an internally connected voltage follower
configuration. Refer to device-specific data sheet for other internal connection options.