eeworldonline.com | designworldonline.com 6 • 2019 DESIGN WORLD — EE NETWORK 47Oscilloscope manufacturers also provide current probes, which
are usually just a combined shunt resistor and differential probe.
These probes also allow the oscilloscope to get the units right, so you
needn’t do Ohm’s law calculations every time you measure current.
However, dynamic range is still limited. Oscilloscopes also have
current-clamp probes, which are limited to around 1 mA resolution.SPECIALIZED EQUIPMENT
A variety of other equipment can measure current, sometimes
while sourcing or sinking current. Equipment of this type includes
electrometers, picoammeters, and Source Measurement Units
(SMUs). These products are specially designed to overcome some
of the standard multimeter drawbacks, and many do have lower
burden voltages and lower input-bias currents. However, the primary
drawback is cost. These devices often employ multiple, more
complicated, active-feedback ammeter sensing methods.
Some current-measurement equipment automatically and
instantaneously selects the shunt resistor to keep the voltage in
range. This approach maintains a maximum burden voltage while also
accurately measuring current. Until recently, this type of dynamically
switching equipment was either too slow (introduced too large of a
dynamic burden voltage) or expensive.
This approach does have two drawbacks, but these can be
mitigated. The first drawback is the shunt resistor switching time,
especially when the current exceeds the range for the current-shunt
resistor value. If the resistor value does not switch quickly enough, the
burden voltage becomes excessive and affects the target device.
The required switching time can be calculated. A simplified
equation, suitable for many practical applications, is:t = C × ΔV / ΔIwhere t = resistor switching time, sec.; C = system capacitance,
F; ΔV = amount of tolerable voltage change, V; and ΔI = currentchange through the resistor. For example, suppose the target system
takes 3.3 V and can tolerate a temporary 3% voltage glitch on a 1-A
change. If the system has 10 μF of capacitance, the required shunt
resistor switching time is:10 μF × 3.3 V × 0.03 / 1 A = 1 μsecThe second drawback is that this approach presents a variable
impedance to the target circuit. Some circuits may exhibit unusual
behavior to a changing supply impedance. However, we can mitigate
this susceptibility by adding decoupling capacitors, which effectively
lower the input impedance at the higher frequencies of interest.
Most modern electronics already require bypass capacitors, so this
drawback is often not a concern when measuring current to target
devices.
This sort of specialized current-measurement equipment must
also account for Johnson-Nyquist noise, the noise any resistor
generates. This noise, the voltage measurement accuracy, and
bandwidth are the critical design factors.
An example of specialized current-measurement equipment
that uses this approach is the Joulescope. It switches shunt resistors
in approximately 1 μsec on over-range to keep the target device
running correctly. It maintains a maximum burden voltage of 20 mV
across the shunt resistor for any current up to 2 A. The Joulescope
is electrically isolated to avoid any grounding and ground loop
concerns.
In addition to being an ammeter, a Joulescope simultaneously
measures voltage so it can compute power (P = I × V) and energy
(E = ∫ P dt).Jetperch LLC
http://www.joulescope.comCURRENT MEASUREMENTS
An expanded display of an
Arduino current waveform
at turn-on, top, revealing
details of the turn-on
sequence. The bottom
display depicts a view of
the Arduino current draw
that includes the max and
min measured current (red
lines) and the average
current (yellow line). Both
displays were generated
by a Joulescope which uses
switched current-sense
resistors to accurately
gauge current.Jetperch — Test and Measurement HB 06-19 copy.indd 47 6/7/19 2:06 PM