User manual
The pseudo-random clock used in the MAX4239 also helps to reduce the effects of intermodulation
distortion as AC signals approach half the chopping frequency (10-15KHz).
This remarkable DC performance allows the µCurrent to have insignificant output offset errors, and
will display zero volts output for a zero current input.
It is also quite low power, around 600uA, thus allowing for battery operation from a small lithium
battery. The supply voltage is specified down to 2.7V, making it ideal for operation from a single 3V
lithium battery.
The MAX4239 also has a companion device, the MAX4238. The only difference is that the MAX4239
is a high bandwidth “decompensated” version of the MAX4238. The MAX4239 requires a minimum
gain of 10 which we have in this circuit, so it’s better to use the higher bandwidth device.
If you want to use the MAX4238 then that is possible without any circuit changes, only the bandwidth
and other AC performance measurements will differ.
A fixed gain of 100 is defined by precision resistors R5, and R3+R11. These are 0.1% low ppm
resistors that are accurate with negligible temperature drift.
R9 ensures output stability, as well as providing a useful jumper link for the single sided PCB layout.
This value will be low enough to ensure error free operation with meters greater than around
100Kohm input impedance. If for some reason your meter is lower than this, than you’ll have to lower
the value of R9 appropriately.
Current Ranges
There are 3 current ranges that are defined by the shunt resistor on each range, and the gain of U1.
R2 (10K 0.1%) is the shunt resistor for the nA range and is permanently connected across the input
terminals. It gives a burden voltage of 10µV/nA (1nA * 10K). The other shunt resistors R1 and R8 are
disconnected in the nA range.
The purpose of having R2 permanently connected is to ensure that the input is not left open circuit.
R8 (10R 0.1%) is switched in parallel with R2 in the µA range by SW1B which gives a burden voltage
of 10µV/µA (1µA * 10R). R2 contributes a small error of less than 0.1% in this case.
R1 (10mR 0.5%) is switched in parallel with R2 in the mA range by SW1B which gives a (resistor)
burden voltage of 10µV/mA (1mA * 10mR). Because R1 is such a low value, the solder joints and
PCB tracks can contribute large errors, so a special purpose designed “shunt” resistor is used. This is
a 4 terminal device that includes the 10mR resistor and two “sense” terminals connected directly
across the resistor on the substrate. This eliminates any error caused by solder joint or PCB trace
resistance.
However, the mA range is special because the shunt resistor is such a small value compared with the
resistance of the range switch, that the switch will dominate the actual total burden voltage. The
switch contact resistance is rated at 70 millohms maximum, so the actual burden voltage on the mA
range will vary from unit to unit, and will change with time, but can be taken as a nominal 70uV/mA.
The maximum current in the mA range is a nominal 300mA, as this is the contact rating of the switch.
But in practice it can be higher than this.