PicoScope 4444 differential oscilloscope example applications
High-resolution differential USB oscilloscope
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PicoScope 4444 differential oscilloscope example applications
There are many measurement applications that benefit from a differential oscilloscope. Listed below are a few examples.
Three-phase load measurements and balancing
It is good practice to ensure phases are equally balanced. The waveform shown was captured using the PicoScope 4444 and four TA300 current probes. The probes are identified and powered by the oscilloscope making them ideal both for short-term and long-term monitoring of load balance.
The phase represented by the red trace is drawing about 17% more current than the other two. The effect of this can be seen in the current flowing in the neutral line (bottom trace).
Power quality - spikes, noise, dips and interruptions
Disturbances on the mains supply can cause a variety of problems ranging from computers crashing to, in more extreme cases, equipment damage.
The PicoScope 4444 is ideal for the long-term monitoring of single-phase and three-phase systems to spot and record any variation from the norm.
The waveform opposite shows the voltage of a three-phase supply (plus the neutral return current). If the voltage waveform crosses into the shaded area, the alarm function beeps and the waveform is stored for later review.
On long timebases PicoScope automatically switches into streaming mode, allowing you to acquire long records that are not limited by the size of the oscilloscope's capture memory.
Tests can be run for days or even weeks and the resulting high-resolution waveforms help identify whether mains quality is a factor.
The timestamp of saved problem waveforms often helps identify whether the problem is internal (which can be an early indicator of equipment failure), external but local (such as a nearby factory) or a generation issue.
Switch mode power supplies are notoriously difficult to troubleshoot and characterize using grounded input oscilloscopes, as much of the circuitry is floating or electrically isolated and often at mains voltage levels.
The PicoScope 4444 differential inputs give you the confidence to probe across circuits and components without concern about shorting floating voltages to ground.
High resolution and vertical zoom capability allow visualization and measurement of small voltage differences on higher-voltage nodes. For example, you can view high-speed gate drives and voltages across current-sense resistors, with no common ground connection to cause short circuits.
A range of voltage and current probes allow visualization of power waveforms in all stages of the power supply.
Differential signals have higher noise immunity and can be transmitted over longer distances than single-ended ones. Applications ranging from balanced audio through to serial data communications benefit from being measured with a differential input oscilloscope.
In the example shown, we have captured an automotive CAN bus waveform. CAN bus uses differential signalling to help withstand the high level of electrical noise found in the engine compartment. The blue and red traces show CAN low and CAN high respectively. The green trace is a differential measurement across CAN high and low which has then been decoded to show the data contained.
There are several advantages of making a differential measurement over using two channels and then using A-B math:
Only one channel is required per differential pair, so you can capture and decode up to four different serial buses.
The full dynamic range, and therefore the full resolution, of the scope can be used to measure the difference between the high and low lines. There is no need to select a larger input range to include the common mode signal, which is blocked by the differential input.
The high common mode rejection ratio (CMRR) effectively removes the common mode noise, ensuring that the signal you are seeing matches that seen by the CAN bus receiver.
AC power to homes, factories and offices is normally delivered at a frequency of 50 or 60 Hz, depending on the region of the world where you live. Power generation companies are obliged to deliver “clean” sinusoidal supply voltages within certain limits set by national regulatory bodies. If the load presented by the consumer is linear, providing the maximum current is not exceeded, the network will run fine. However, many modern devices do not present linear loads to the AC supply. Instead they take “bites” out of the supply waveform, so the current drawn includes harmonics of the 50 or 60 Hz fundamental supply frequency.
Differential input oscilloscopes measure the difference in signal between their two inputs. This is especially useful when you are trying to measure low-level signals in electrically noisy environments.
In this example we capture a human heartbeat simply by holding one input connector in each hand.
Many electronic devices today are left in sleep mode until instructed to wake up and function as normal. Most TVs are in standby mode until we arrive home and punch the “power” button on the remote control to watch the football game. It is important to measure the current a device is consuming in standby mode to ensure it complies with relevant energy efficiency standards.
This older TV consumes 69 W in standby mode (over 600 kWh per year). More modern TVs consume less than this when operating and under 1 W in standby.
When switch-on occurs we need to observe the power-up timing characteristics, inrush current and other parameters. There is normally a big difference between “standby” and “on”, so measurements must be made with high resolution to match the large dynamic range of the two states. The PicoScope 4444, with 12 to 14-bit resolution, is ideal for making such measurements.
The measurement was made using a PicoConnect 442 1000 V CAT III voltage probe and a TA300 40 A current probe.
Load cells and strain gauges
Many sensors such as load cells, pressure sensors and strain gauges respond by changing resistance and are usually wired in a bridge arrangement.
The 4 wire 1 kg load cell shown here is a typical example. It is excited by a 10 V DC source (in this case from a bench power supply).
The output is a small 1 mV per gram differential signal riding on a 5 V common mode signal. The differential inputs of the PicoScope 4444 allow direct connection to such sensors.
The 14-bit mode allows a good resolution of weight measurement. In this example one voltage range can cover the whole 1 kg range yet still resolve changes of less than 0.1 g.
The fast sampling rate allows transient events to be captured that most data acquisition devices are too slow for.
The waveform shows the effect of dropping a small coin onto the load cell. Once it has settled we can use the rulers to measure the weight as 9.5 g.
Hybrid and electric vehicle (EV) applications
The PicoScope 4444 is ideal for making measurements on hybrid and electric vehicles. Voltage ranges up to 1000 V (CAT III rated) allow direct measurements of battery, inverters, motors and chargers. A range of single and three-phase current clamps measure up to 2000 A.
The deep memory, high resolution and powerful zoom tools reveal hidden details in waveforms.
Away from the high voltage components, the PicoScope 4444 can capture waveforms from sensors and actuators and also decode automotive serial signals (CAN, CAN FD, LIN and FlexRay).
The waveform captured from a Tesla Roadster shows current drawn by the three-phase motor during acceleration from standstill. The brown trace shows current drawn from the 375 V battery pack: peak current is 590 A.