AN839 Datasheet, PDF (1/5 Pages) Silicon Laboratories – Novel Current-Sense Measurement with Automatic Offset Correction

AN839
Novel Current-Sense Measurement with Automatic Offset Correction
Operates at 0.8 V and Runs on a Few MicroAmps and is Accurate to ±1 µA
1. Introduction
Pairing a simple circuit using nanopower analog op amps with a microcontroller can monitor single battery cells
and solar harvesters. Pairing up a 0.65 V/1µA nanopower op amp with the low-power Cypress PSoC3
microcontroller and some simple external circuitry can yield a very low current measurement system, suitable for
monitoring miniature power harvesting solar cells or a single-cell battery. The circuit operates on a few microamps
at 1.8 V; optionally, the whole circuit can be self-powered from the source being measured. The op amps operate
from voltages as low as 0.65 V and are connected directly to the cells; the PSoC3 microcontroller utilizes an
internal boost regulator which operates from sources as low as 0.5 V.
Figure 1. Current-Sense Amplifier with Input Offset Voltage Correction
The current sense amplifier is shown in Figure 1. The analog portion of the circuit operates from as low as 0.65V
and draws 860nA at no load. The circuit provides a 0-500mV output for measured currents of 0-100µA, though the
scale can be adjusted by changing the values of a few resistors. With its extremely low power, the circuit can
simply remain “always on,” providing a continuously monitored, averaged indication of current. The CY8C38
subsequently reads the value periodically, remaining in sleep mode and consuming less than 8 µA.
The analog circuit features the TS1001, a nanopower op amp capable of supply voltage operation to 0.65V
configured to servo P-channel MOSFET Q1 in a current source configuration. The current source, draws current
through R1 to compensate for the voltage drop across R2 caused by the current flow from the IN terminal to the
load. Resistor R3 converts the current to a voltage and C1 provides filtering. The filtering is critical in allowing the
current sense amplifier to continuously provide an averaged current output, enabling the microcontroller to sleep
for long periods and save power, waking only periodically to read this mean current level.
Rev. 1.0 7/14
Copyright © 2014 by Silicon Laboratories
AN839