A New Approach to Characterizing Power in Low Power Devices
Minimizing the power consumption and extending battery life is a major focus for manufacturers of mobile electronics. That requires minimizing the current draw of all integrated circuits and electronic subassemblies. Characterizing these components starts with measuring their current consumption. Many of today’s devices have operating currents of micro-amps or less.
Minimizing the power consumption and extending battery life is a major focus for manufacturers of mobile electronics. That requires minimizing the current draw of all integrated circuits and electronic subassemblies. Characterizing these components starts with measuring their current consumption. Many of today’s devices have operating currents of micro-amps or less.
There are two basic methods for measuring low currents on a low power device under test (DUT):
- A power supply and a DMM: This method of measuring the current flowing from a power supply through a DUT requires placing a precision DMM in series with the circuit and measuring the current with the DMM (Figure 1). Although a DMM with 6½-digit resolution can produce very accurate measurements of the level of current through the device, it can also introduce many problems during characterization due to the voltage burden created by the DMM. Even though the voltage at the output of the power supply may be at the programmed value, the voltage at the DUT is actually lower than the programmed value. Rather than the programmed voltage, the voltage seen at the DUT’s terminals is equal to the programmed voltage minus the voltage across the DMM (VDUT = VSET – VDMM). If this voltage drop is not accounted for and the engineer assumes the voltage at the device is equal to the programmed voltage, then power and resistance measurements will have significant error because the voltage used to calculate them will be higher than the value at the DUT.
Figure 1. Power supply and DMM configured for measuring the current flow through a low power DUT
This drop in voltage also causes problems when testing the device at voltages near the minimum operating voltage. If the voltage burden of the DMM is too great, the voltage seen at the device may be below the minimum operating voltage, so the device won’t respond normally, resulting in incorrect measurements. This drop in voltage could be compensated for by outputting a higher voltage at the power supply so the desired voltage appears at the DUT. However, the voltage burden created by the DMM varies with the amount of current flowing, so compensation is difficult.
A second DMM could be used to measure the voltage at the device directly, but this not only adds cost and complexity to the test system by introducing another piece of equipment, it can represent a significant source of error for low current measurements. The DMM adds additional load to the test circuit, resulting in currents higher than those actually flowing through the device.
- A precision measurement power supply: Some power supplies not only source power but have built-in measurement capability. The latest generation of readback power supplies support measuring the current through a device with the same precision as a high quality 6½-digit DMM but far more simply because only a single instrument is involved (Figure 2). With no need to integrate, synchronize or program multiple instruments, testing can begin sooner.
Figure 2. Measuring current with a precision measurement power supply
Precision measurement power supplies can measure both the current and voltage applied to the DUT. Current is measured internally, so it doesn’t place a voltage burden on the test circuit like a series DMM would; the voltage at the device is equal to the programmed voltage.
For even greater accuracy, voltage can be sensed directly at the device using remote sense leads placed at the device terminals, allowing the precision measurement power supply to compensate automatically for any voltage drops across the test leads that supply current to the device. These sense leads have very high input impedance, so they place virtually zero load on the test circuit. Using these techniques and features of a precision measurement power supply, design engineers can characterize devices at almost any current level very accurately.
To illustrate how a precision measurement power supply can be applied to low-power device characterization, let’s consider measuring the current consumption of a precision voltage reference when nothing is connected to the reference’s output. Let’s say the datasheet for this voltage reference device specifies the quiescent supply current at a typical level of only 31μA and a maximum level of 35μA. As illustrated in Figure 3, test connections are extremely simple; only two test leads, HI and LO, are required. Remote voltage sensing is not required for accurate voltage measurements because the currents are very low, so they won’t produce significant voltage drop in the test leads. Shielded cabling is recommended to reduce noise. If the test circuit is grounded, grounding should be at a single point to avoid measurement error due to ground current loops.
Figure 3. Test connections from a precision measurement power supply to the precision voltage reference under test
Configuring the instrument for maximum accuracy
To measure currents in the low micro-amps range accurately, the precision measurement power supply must be properly configured:
- Set resolution to 6½ digits
- Enable auto-zeroing. This automatically measures an internal reference to zero the instrument for each triggered measurement.
- Set the number of power line cycles to 15 to increase both measurement resolution and accuracy.
- Configure the filter settings. When the averaging filter is used, the instrument will return readings that are the average of several measurements. Averaging increases measurement stability. Configure the trigger settings sample count to match the filter count.
- Configure the source delay. For low current measurements, be sure to allow enough time for currents in the test system to settle to their final values. Although 10ms is adequate for most micro-amp level measurements, a longer source delay may be necessary if the DUT has a lot of input capacitance or there is an external filter capacitor on the fixture.
Running the Test
To begin the test, first set the output voltage to the proper level for the DUT. Next, set the current limit to a value low enough to protect the DUT but high enough that sufficient current will flow for the device to operate. Finally, we turn the output on to begin taking measurements. Figures 4 and 5 illustrate the results.
Figure 4. An example of low current measurements performed on a precision voltage reference DUT using a Keithley Series 2280S Precision DC Power Supply. Measurements are stable down to approximately the 100nA digit. The statistics displayed at the bottom of the screen show a very low peak-to-peak value and standard deviation for the readings.
Figure 5. Series 2280S low voltage measurements on the precision voltage reference
