Figure 145. Pga Mode, Internal Gain Setting (X2/X4/X8/X16), Inverting Input Used For Filtering; Calibration - ST STM32L4x6 Reference Manual

Operational amplifiers (OPAMP)
Programmable Gain Amplifier mode with external filtering
The procedure to use the OPAMP to amplify the amplitude of an input signal, with an
external filtering, is presented hereafter.
•
configure OPAMODE bits as "internal PGA enabled",
•
configure PGA_GAIN bits as "internal PGA Gain 2, 4, 8 or 16",
•
configure VM_SEL bits as "GPIO connected to VINM",
•
configure VP_SEL bits as "GPIO connected to VINP".
Any external connection on VINP can be used in parallel with the internal PGA, for
example a capacitor can be connected between VOUT and VINM for filtering purpose
(see datasheet for the value of resistors used in the PGA resistor network).
Figure 145. PGA mode, internal gain setting (x2/x4/x8/x16), inverting input used for
1. The gain depends on the cut-off frequency.
20.3.5

Calibration

At startup, the trimming values are initialized with the preset 'factory' trimming value.
Each operational amplifier offset can be trimmed by the user. Specific registers allow to
have different trimming values for normal mode and for low-power mode.
The aim of the calibration is to cancel as much as possible the OPAMP inputs offset voltage.
The calibration circuitry allows to reduce the inputs offset voltage to less than +/-1.5 mV
within stable voltage and temperature conditions.
For each operational amplifier and each mode two trimming values need to be trimmed, one
for N differential pair and one for P differential pair.
There are two registers for trimming the offsets for each operational amplifiers, one for
normal mode (OPAMP_OTR) and one low-power mode (OPAMP_LPOTR). Each register is
composed of five bits for P differential pair trimming and five bits for N differential pair
trimming. These are the 'user' values.
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filtering
DocID024597 Rev 3
RM0351