AN833

Rev. 1.1

3

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:

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.