7 Hints for Precise Current Measurements

Table of Contents:

• Introduction
• Hint #1 – How to Choose the Right Current Sensor
• Hint #2 – Effective Connection to Your DUT
• Hint #3 – Ground Lead Connection for Accurate Measurements
• Hint #4 – Understanding Effective Measurement Bandwidth
• Hint #5 – Influence of the Current Sensor Insertion
• Hint #6 – Applying User Calibration
• Hint #7 – Low Current Measurement Tips

Introduction

The CX3300 series of Device Current Waveform Analyzers can visualize wideband low-level, previously unmeasurable, or undetectable current waveforms. Measuring low-level dynamic current can be challenging due to noise or the bandwidth issues, but the CX3300 solves these issues without compromising noise and bandwidth.

However, in order to take full advantages of the instruments’ capabilities, there are some important points to note such as how to select the right current sensor for your application. This application note explains 7 hints that can be undertaken in order to obtain more favorable current measurement with the CX3300.

Current Sensor (CX1101A, CX1102A, CX1103A) Overview

As shown at Figure 1, the CX3300 is configured with a dedicated mainframe and three sensors, CX1101A, CX1102A and CX1103A. These sensors are differentiated by bandwidth, current coverage, and sensitivity, to satisfy various measurement needs.

CX1101A: This sensor is the basic current sensor and covers wide measurement ranges from 40 nA to 1 A and operates under the common mode voltage up to ± 40V. The maximum bandwidth is 100 MHz (sensor standalone). With the CX1206A dedicated sensor head, the CX1101A can measure up to 10 A.

CX1102A: As with the CX1101A, it covers wide measurement ranges from 40 nA to 1 A, and the maximum bandwidth is 100 MHz (sensor standalone), however the maximum common mode voltage is ± 12V. This sensor provides dual range, primary and secondary ranges, to support a measurement across a wide dynamic range. Note that the secondary range is fixed as 1/100 of the primary range1 , and each range occupies the measurement channel respectively.

CX1103A: The CX1103A supports the series’ minimum measurable current, and the series’ maximum bandwidth. This sensor must be placed at the low side of the DUT (circuit common).

Current Sensor Technology

There are several methods for measuring the current and they are classified into two groups according to the connection type: contact or non-contact. Generally, a current sensing method with contact has higher sensitivity, and the CX3300’s three current sensors employ it. The major conventional contact current sensing method is to measure the voltage between a shunt resistor and calculate the current from the measured voltage and the shunt resistor as shown at Figure 2a. There is a trade-off between the sensitivity and the bandwidth, and the higher sensitivity can be achieved using a larger shunt resistor, while the bandwidth is narrower with a larger shunt resistor.

In contrast, the new technology used in the CX3300’s current sensors, as shown at Figure 2b, enables you to maintain very good low current measurement sensitivity across a wide frequency range. The CX3300 mainframe works in tandem with the sensors, and they are both optimized for the measurement of current.

Sensor Head Overview

CX1101A and the CX1102A can be used with a variety of different sensor heads to provide a variety of connectivity as shown in Table 1. The coaxial type connection with SMA connector(s) can provide low noise and wide bandwidth performance. For a quick measurement, the sensor head with a pair of test lead connectors (CX1205A) or a twisted pair cable (CX1204A) should be used. The CX1206A, which is the dedicated sensor head for the CX1101A, has an expander box enabling up to 10 A current measurements using a pair of banana cables.

Note that the CX1103A has a fixed sensor head with the SMA connector to provide the best performance in the bandwidth and low current measurement capability.

Hint #1 – How to Choose the Right Current Sensor

Step 1: Check the measurement current range and required bandwidth

In order to characterize the dynamic current, the most important thing is understanding the current range and required bandwidth for the measurement.

Three types of current sensor are available for the CX3300A and all current sensors have multiple current ranges and the range can be easily changed on the front panel.