measurement setup

Measurement Setups using the RocketLogger

This page describes three typical measurement setups using the RocketLogger and important considerations for setting up your own measurements. For configuration and controlling the actual measurement, checkout the RocketLogger Measurement Control page.

Important Measurement Considerations

Carefully read and understand the following considerations to perform correct measurements and avoid permanent damage to the RocketLogger.

Current Channels

• The LO terminal (outer conductor) of both current channels are shorted together and directly connected to the internal ground of the RocketLogger. The resistance between both terminals is < 3 mΩ and not protected against overload. If the RocketLogger is connected to mains earth, these connectors are as well.
• The HI terminal (inner conductor) of both the current channels are virtually connected to ground. Both of these inputs are fused independently with a 046702.5NR 2.5 A fuse.
• The RocketLogger's ground plane (and therefore the current LO terminals) is connected to the Ethernet shield on the BeagleBone, and may be connected to mains earth. This can result in short circuits or ground loops trough the network shield and/or the power supply when the device under test is connected to a computer or other networked devices. This can be tricky and the connection can go through many devices like network switch, USB hubs, networked power supplies, etc. To circumvent problems related this this, we recommend the use of Ethernet connectors that do not connect to the shield on the RocketLogger side.
• Note that the crosstalk between the current channels can sometimes be significant, when measuring very high and very low currents at the same time.

Voltage Channels

• The voltage channels have a single measurement connection. The measurement is referenced to internal ground (i.e. the current LO terminals) for all channels.
• The voltage inputs (inner conductor) have an extremely high input impedance, but only as long as the inputs stay within the measurement range of ±6 V of the ground.
• The voltage guards (outer conductor) are actively driven by the internal buffer amplifier for reduced leakage. Do not connect anything to them and do not short them together -- this can permanently damage the voltage channels.
• For input voltages outside the allowed voltage range, the voltage channels have a (differential) input resistance of 10 kΩ.

General Remarks

• Use probes and cables where both the inner and the outer conductor are isolated.
• Use low resistance cables for high current measurements.
• Only connect one of the current LO terminals or digital GND to the same potential to avoid ground loops.
• When large currents are flowing through the RocketLogger current probes, you should use one voltage probe to measure the voltage drop across the ground connection and subtract it from the voltage measurements to get more precise readings. This can also be achieved by keeping the reference GND (current LO terminal) directly connected to the device under test, but this might not always be possible.
• Use a sampling rate of at most 16 kSPS for the best ADC performance.
• Digital interfaces and other measurement equipment can draw significant currents. Disconnect them if necessary.
• Try to keep the area of any loops formed by the probes as small as possible. Even small, unshielded segments can result in noticeable interference, usually from the 50 Hz grid frequency.
• Keep RF transmitters away from the RocketLogger and its probes, whenever possible. WiFi dongles should be connected over an USB cable and kept at least ~40 cm away from the RocketLogger.
• The Ethernet shield of the BeagleBone is connected to the internal GND. Therefore, when using multiple networked loggers or measurement equipment (e.g. computer, networked lab supply) physically connected to the same switch/router, ground loops or short circuits may occur through this hidden current path. Workaround: use Ethernet connectors that do not connect to the shield on the RocketLogger side.

Typical Measurement Setups

Single Channel Measurement

The power measurement of a single consumer is straight forward. The current LO terminal (internal ground reference) of the current channel is connected on the consumer side: for the low-side measurement (recommended) to the circuit ground (GND) as shown on the left for Device A. When performing high-side measurements, it is connected to the supply of the circuit (VCC) as shown on the left for Device B. The current HI terminal is connected to the supply voltage Vsup or ground GND of the power supply, for high- and low-side measurements, respectively. The single ended voltage input is connected to the other supply connection to measure the circuits exact voltage supply, which is referenced to the RocketLogger internal ground, i.e. the current channel's LO terminal.

In post-processing, the (merged) current and voltage channels are multiplied component wise to calculate the circuit's power consumption. The above shown low low-side measurement setup results in current I2 output that has reverse polarity. When calculating the power consumption, the component wise multiplication results have to be multiplied by -1. In the high-side measurement setup, both I1 and V1 outputs have reverse polarity. The component wise calculation of the power consumption however has the correct sign, because the negative signs of both measurements cancel out.

Parallel Measurement of Independent Subsystems

For parallel measurement of two independent circuits, the single channel measurement is repeated on both current channels as long as the power supplies of the circuits are decoupled. Keep in mind that the current LO terminals share the same potential across both measurements circuits, which is typically not a problem when measuring independent circuits with isolated power supplies. Even high and low side measurements setups can be combined as shown in the figure above.

If the circuits share the same power supply or use non-isolated power supplies, e.g. shared GND terminal or connected to mains earth, the setup for connected subsystems discussed below has to be used.

Split Measurement of Connected Subsystems

For split current measurement of internally or externally connected subsystems, the single circuit measurement is repeated on both current channels. However, because of the shared GND (or VCC) connection on the RocketLogger side, only one of the current LO terminals is connected to the power supply side. A low-side measurement setup is recommended due to its flexibility and support for independent supply voltages for each of the subsystems. In this setup, (only) one of the LO terminals is connected to the common ground of both systems. The HI terminals are connected to the GND of the corresponding subsystems.

However, a low side measurement setup can be difficult for fixed hardware designs as often a shared ground plane is used. In this case, a high-side measurement setup for subsystems with equal supply voltage can be used. In the high-side measurement setup, one of the current LO terminal is connected to the common supply voltage Vsup. The HI terminals are connected to the VCC of the corresponding subsystems.

For both measurement setups, a minimal drop across the current channels is expected (see datasheet). When measuring only small currents, one might neglect this impact and measure only the total supply voltage of the subsystems (V1, and V2 for low-side setup). For highest accuracy, it is however recommended to additionally measure the voltage drops across the current channels using two additional voltage channels (V3 and V4 in the setups above). In the analysis, one will use the difference of the total voltage and the individual voltage drop to get the exact subsystem supply voltage.

Chain of Subsystems Measurement

To measure the power flow in a typical energy harvesting system, one has to deal with three connected subsystems: a harvesting source (e.g. a solar panel), a harvesting management circuit (converter), and a consumer (the supplied application). To measure the current flow in this subsystem with typically different voltage levels in each subsystem, a low-side measurement setup is required. To avoid any ground loops invalidating the measurement, the GND of the intermediate converter subsystem has to be connected to one of the LO terminals. The current HI terminals are connected to the GND of the source and the consumer circuit's GND. The corresponding two voltage levels on the source and application side are also recorded to complete the power measurement. Again, it is recommended to measure the voltage drops across the current channels using two additional voltage channels for highest current measurement accuracy (V3 and V4 in the diagram).

Due to reversed polarity, the sign of the source current channel (I1) has to be inverted in post processing for correct current flow calculations from source to consumer.

Troubleshooting your Measurement Setup

We document a few typical issues related to the measurement setup and how the can be fixed or reduced.

1. The circuit under measurement behaves differently when the RocketLogger is attached.

   Even though the range switching on the RocketLogger is very fast, it's still possible that it affects the circuit under measurement. You can force the RocketLogger into the high current range to see if the range switching is the cause of the problem. The DC burden voltage may also cause some problems, especially if high resistance probes are used.

2. There is a large amount of 50 Hz / 100 Hz / 150 Hz interference on the signal.

   Due to the low noise floor of the RocketLogger, even a small pickup of interference is noticeable. The pickup is commonly caused by ground loops (see Measurement Recommendations) or 'antenna leads'. The best way to avoid the later type of interference is shielding the entire measurement setup. For this purpose, the current probes of the RocketLogger are shielded and the voltage probes guarded. However, the circuit under measurement has to be shielded as well. If complete shielding of the circuit is not possible, you can use twisted cables and try to keep the loop areas as small as possible.

For other issues regarding the measurement setup, we refer you to the freely available "Keithley: Low Level Measurements Handbook" with additional information on precision measurements.

Measurement Channel Input Structure

Simplified Current Channel Diagram

Simplified Voltage Channel Diagram

For more detailed circuit info, we refer to the hardware design data.