AN833 Datasheet, PDF (3/5 Pages) STMicroelectronics – FREQUENCY MODULATION ON L4981B

AN833
Figure 3. Ultra-Low Supply Current Amplifier Circuit with Amplifier Offset Voltage Correction
Figure 3 shows an alternative configuration which provides a means for calibrating out the amplifier offset voltage.
The circuit is set up for measuring very low currents, in this case from three series-connected PIN solar cells. The
circuit can be used as part of an energy harvesting system in a maximum-power-point-tracking (MPPT) scheme,
where the microcontroller adjusts the loading of the cells to maximize V x I and yield the maximum available power.
The circuit works in a similar way to Figure 1, but it has been scaled for a selectable 100 µA/500 µA full scale
current where the full-scale sensed voltage across R2 has been decreased to 50 mV to improve efficiency. With
this lower voltage, the ±3 mV specified input offset voltage of the TS1001 causes up to a 6% offset error, which
may be unacceptable.
This offset may be calibrated out using the principal of making two measurements of the same parameter at the
two gain settings. The offset then can be found as:
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Microcontroller-based systems power budgets have dropped dramatically in recent years, enabling these systems
to be powered from sources such as coin batteries and miniature solar cells. Analog circuits such as this
micropower current-sense amplifier, which remain always on, draw very little power from the power source, and are
becoming increasingly useful to save yet more system power by allowing the microcontroller to remain sleeping
while the circuit stays awake monitoring and averaging battery parameters. For additional information, see the
TS1001 Op Amp documentation.
Rev. 1.1
3