User manual

You will notice that the virtual ground is connected to the sense side of R1, this means that the sense
voltage for R2 and R8 also flow through this terminal, but the sense current in these cases is
negligible so it has no effect.
SW1A selects which shunt resistor gets fed through to the amplifier.
Power Supply
Any current adapter must be able to handle both positive and negative inputs, and thus a dual polarity
power supply is required. In a battery powered device, this can be achieved in one of three ways.
The first is by using two or more series batteries providing a middle “0V” tap. This is very convenient,
but takes more space, there are more batteries to replace, and you can get uneven drain from the
batteries thus making true low battery detection more difficult.
The second is by using a single battery supply and generating a negative supply using say a switched
capacitor inverter. This is convenient for low current applications, but it generates noise and requires
filtering. Also, using a 3V lithium battery means a total power supply voltage from 5.4V to over 6V. But
our MAX4239 can only handle a maximum 5.5V supply voltage, so extra diodes would be required.
The third method involves a “virtual ground” split supply circuit, and this is what is used in the
µCurrent. The circuit comprises U2, R6, and R7. R6 and R7 split the battery supply in half, and U2
buffers this to provide a low impedance output.
U2 is an LMV321, a general purpose low power low voltage op-amp, essentially a low voltage version
of the venerable LM351. Power consumption is a measly 130uA, with R6 and R7 taking another
13.5uA or so.
R10 is used to ensure output stability.
The output from R10 is now the “virtual ground” reference for the rest of the circuit. Thus ensuring U1
has a +/-1.5V supply from the battery, and the input current shunt resistors can now sense current in
either direction.
Low Battery Detection
To ensure that what you read on your multimeter is accurate, it is important to know if the battery
voltage is low and thus possibly affecting the measurement.
U3 does this job very neatly and accurately in a single chip. It is a Texas Instruments TPS3809L30
“Supply Voltage Supervisor”