When AC voltage-based complex-impedance measurements are made, the system applies the signal, then measures the in-phase and quadrature-phase voltage and current. From these, the capacitive, inductive and resistive components of the measurement are determined.

By choosing from alternative frequencies, the impedance of the measured component can be optimized compared to parallel impedances for best measurement results.

In some cases, capacitor polarity can be tested by applying current-limited DC to the capacitor and measuring the voltage across the capacitor. A lower voltage is developed if the capacitor is installed incorrectly.

To eliminate the effects of path resistance internal to the tester and in the fixture wiring, measurement points can be externally sensed, providing 4-wire Kelvin measurement capability at the UUT rather than internal to the tester. This helps increase the accuracy of low impedance measurements.

To prevent semiconductors from turning on during the measurement, you can choose a 20 or 200mV full range stimulus in place of the standard 2V stimulus. AC measurements can be biased to prevent interference from parallel diodes in the measurement path.

Guarding
In-circuit measurements often contain parallel impedances that can cause measurements to deviate from component nominal values.

Guarding provides the capability to minimize the effects for parallel impedance paths. Guarding uses special sense and drive circuitry to source or sink current into other UUT circuit nodes to eliminate current flow through these paths.

CheckSum Analyst systems allow you to specify multiple guard points during a measurement. Guard points can be externally sensed to provide additional guarding accuracy when low impedance paths exist. Any test point can be used as a guard point. No special wiring is required.

Unlike other solid state switch-based in-circuit test systems, CheckSum uses separate guard drivers for each of its guard points. This additional sophistication can provide the proper guard voltage at each guard point, regardless of connection impedance differences.

Even without guarding, Analyst systems can usually directly measure components of different types connected in parallel, such as a capacitor and a resistor, with the use of complex-impedance measurements.

Transistors and FET Testing
Three-terminal devices such as transistors and FETs are tested by measuring between the current-carrying terminals while biasing the control terminal. FETs are biased with voltage, while transistors are biased with current.

Opto-Isolators
Opto-isolators can be tested by sourcing into the input leads while measuring the output impedance. By testing each device in the on and off state, high confidence is obtained.

Diode Testing
Diodes, LEDs, zener diodes and transistor junctions are tested by applying a constant-current, then measuring the voltage dropped across the device.

IC Presence and Orientation Testing
IC presence and orientation is verified by checking the semiconductor junction voltage of the protection diodes typically present between IC pins and the UUT power supplies. This mapping is self-programmed from a known-good UUT.

Opens and Shorts Detection
Since most faults that occur during manufacturing are shorts, MDAs provide the ability to perform continuity testing for opens and shorts. The systems self-learn the continuity map of a known-good UUT, then test against this map for other UUTs. Selected open/short measurements can be ignored to prevent testing of components near the continuity threshold or to provide better diagnostics with separate measurements.

Using TestJet Technology*, Analyst in-circuit test systems can find open connections to surface-mount technology (SMT) devices such as ICs and connectors.

Relay Testing
Up to 24V with up to 100mA can be used to actuate relay coils. This allows testing of contacts in each state to ensure that the contacts are not shorted, and that the coil is operational.

Transformer Testing
Transformer coil presence can be tested with resistance and/or inductance measurements. CheckSum MDA systems can also test the polarity of transformer connections to ensure that they are correct. Since transformers are often hand terminated, this will find faults not detected by normal coil resistance testing.

TestJet Technology
A common fault in surface-mount technology manufacturing is open connections. On components with bussed connections or high impedance pins, these faults cannot be detected by normal analog in-circuit measurements.  Analyst in-circuit testers allow you to detect these faults using award-winning TestJet Technology. A flat probe is built into the fixture over each component body to be tested. The system measures from this top probe to each signal pin on the SMT device. By measuring precise capacitance values (in the fF region) the system detects open connections. This technology works for most SMT ICs and connectors.

Capacitor Polarity based on TestJet Technology
In some cases, constant-current and voltage measurements of a polarized capacitor can be used to detect incorrect polarity since the capacitor draws additional current as the voltage increases in the incorrect polarity. As a practical matter, this technique cannot be used in many cases during in-circuit testing because of voltage or parallel impedance limitations. In this case, the SMT option can be used to detect the polarity of most axial/SMT aluminum and tantalum capacitors up to about 200µF.

Power Supplies and Relay Switches
The Analyst ems+ft system power supply options can be used to power-up your UUT:
 
• PS-UUT-2 Power Supply option
2 programmable supplies, 0 to 60 VDC, up to 12.5A
 
• PS-1 Fixed Power Supply and Relay Switching option
Each PS-1 supply and ground is relay-switched under program control. Fixed power supplies include:
+5VDC @ 8A
+12VDC @ 2.5A
-12VDC @ 1A
+24VDC @ 2.5A
24VAC (line frequency) @ 0.75A.
Each PS-1 supply and ground can be relay-isolated from the UUT.  In addition to the System power supply options, the optional PWR module provides switched +5VDC @ 1A, +12VDC @ 1A and -12VDC @ 0.1A, available at the fixture interface panel. Variable DC supplies include ±10VDC (600 ohm source impedance), ±12VDC (100mA source current), and -10mA to +10mA constant current. The PS-1 power supply can provide 24 additional, individually programmable, 1A Form-A power relays that can be used for switching power and signals at the fixture interface panel.