Application Note | Optimal Capacitance and AC Impedance Measurements Using the 4200A-SCS Parameter Analyzer

Both the 4215-CVU and 4210-CVU are multi-frequency (1 kHz to 10 MHz) AC impedance measurement modules for the 4200A-SCS parameter analyzer (see Figure 1), allowing users to easily perform CV measurements. The difference between the two CVUs is the number of test frequencies and the AC drive voltage . The 4215-CVU has 10,000 different frequencies with a resolution of 1 kHz; the 4210-CVU has 37 different frequencies. The AC drive voltage range of the 4215-CVU is 10 mV to 1 V rms, and the AC drive voltage range of the 4210-CVU is 10 mV to 100mV rms.

Figure 1. 4200A-SCS Parameter Analyzer

A simplified 4210-CVU and 4215-CVU model is shown in Figure 2. The capacitance of the device is determined by sourcing an AC voltage and measuring the AC current and phase while a DC voltage is applied or swept across the device.

Figure 2. Simplified CVU diagram

The time domain AC value is processed into the frequency domain to generate the impedance in phasor form. We can calculate the device capacitance from the AC impedance and the test frequency using the following formula:

The CVU measures capacitance using the Automatic Balancing Bridge (ABB) method. The ABB is used to cancel an AC signal of known frequency at one terminal of the DUT (LPOT if the AC ammeter is on LCUR) to guard against stray impedance. This AC ground holds the LPOT of the CVU at 0 VAC so that all AC current in the test circuit flows to the AC ammeter and not through any shunt capacitance in the test circuit.

Depending on the test settings, including frequency, AC drive voltage and current range, the CVU can measure capacitance from pico-farad to millifarad . The user-specified test range depends on the device under test and the derived parameters. The test frequency range is 1kHz to 10MHz. The DC bias function is ±30V (60V differential).

The typical model for DUT measurement is usually a series or parallel resistor-capacitor (RC) circuit. As shown in the simplified model in Figure 3 , the CVU can measure the DUT as a series configuration (RSCS) or a parallel configuration (RPCP).

Figure 3. Simplified measurement model

Figure 4. Vector diagram of impedance.

By using the Clarius built-in Formulator tool, other parameters such as inductance can also be easily extracted from the measured data. The impedance vector diagram in Figure 4 shows the basic formula for impedance.

As shown in Figure 5 , a CV measurement system can be quite complex because the configuration includes measurement instruments and software, signal path wiring, test fixtures, and devices. To make optimal measurements, the test setup and timing parameters of the CVU must be set accordingly. Appropriate cables, probes, and test fixtures must be used, and then connection compensation must be performed. Finally, the device itself can cause measurement problems. The following sections discuss the hardware and software considerations for making good capacitance measurements.

Figure 5. CV measurement system

This section describes how to use the appropriate cables and connections , how to protect the chuck and device terminals , and how to configure the AC ammeter terminals .

To obtain the best measurement results, only use the included red SMA cable to connect to the CVU. The included accessories are as follows:

● 4 CA-447A SMA to SMA 1.5m cables (red)

● 4 CS-1247 SMA to BNC adapters

● 2 CS-701A BNC T-type fittings

● 1 torque wrench to tighten the SMA cable connection

Figure 6. CVU connection diagram for 2-wire sensing

The included accessories can be connected to the test fixture or probe via BNC or SMA connections. Figure 6 shows the CVU and the included accessories configured for 2-wire sensing. The CS-1247 SMA to BNC adapter is connected to each CA-447A SMA to SMA cable. The HCUR and HPOT terminals are connected via the CS-701A BNC T-fitting to form the CVH; the LCUR and LPOT are connected together to form the CVL. Using the included torque wrench, tighten the SMA cable connections to ensure good contact. The red SMA cable is 100 Ω . Two 100 Ω cables in parallel are 50 Ω , which is the standard configuration for high-frequency source measurement applications.

Figure 7. Correct connection from CVU to DUT

Figure 7 is an example of 4-wire sensing of the DUT. In this example, the HCUR and HPOT terminals are connected to one end of the device, and the LPOT and LCUR terminals are connected to the other end of the device. To improve bandwidth, we connect the outer shield of the coaxial cable to the metal test fixture. We use a 4-wire connection to the device to simplify sensitive measurements by sensing the voltage as close to the device as possible.

The outer shield of each of the four coaxial cables must be connected as close to the device as possible to minimize the loop area of ​​the shield. The outer shield of the coaxial cable should also be connected to the metal test fixture to reduce noise and coupling from external sources. This reduces inductance and helps reduce resonance effects, which can be a burden at frequencies above 1MHz.

Figure 8. Ground jumper connecting the common of two control devices

Figure 8 shows a jumper connecting the common portion of the two probe cable assemblies. Keithley Instruments has a line of 4210-MMPC multi-measurement cable kits that are suitable for a variety of probes and can make common connections to a variety of control devices.