Measuring Principle Alike the other MDAs (Manufacturer Defect Analyzer), the board under test is judged to be pass or fail as compared with its reference value that was learned from a known good PC boards. Generally, the measuring section in our tester is classified into three methodical sources. DC-Constant Current measurement (DC-CC) Fig. 1 gives an outline of measuring Device (Z) using DC-CC measuring method. In short, Voltage (Vm) appears across the circuit is measured out after Constant Current ( I) was applied there. In our tester, this DC-CC measuring method is normally used for measuring Resistors of less 400 ohm, Capacitors of over 4F and Forward voltage (Diodes, Transistors, ICs, etc.) Resistors Resistance value (Z) is calculated by DC Constant Current (I) and Voltage (Vm). Capacitors Capacitance value (Z) is calculated by DC Constant Current (I), Measuring time (T) and Voltage (Vm). Diodes (VF) Forward voltage (VF) is Voltage (Vm) measured by applying DC-Constant Current (I). DC-Constant Voltage measurement (DC-CV) Fig. 2 gives an outline of measuring Device (Z) using DC-CV measuring method. In short, Current (Im) appears across the circuit is measured out after Constant Voltage ( V) was applied there. In our tester, this DC-CV measuring method is normally used for measuring Resistors of over 400 ohm. Resistors Resistance value (Z) is calculated by DC Constant Voltage (V) and Current (Im). AC-Constant Voltage measurement (AC-CV) Fig. 3 gives an outline of measuring Device (Z) using AC-CV measuring method. In short, Current (Im) appears across the circuit is measured out after Constant Voltage ( V) was applied there. In our tester, this AC-CV measuring method is normally used for measuring Capacitors of less 4F and Inductors. Resistors Resistance value (Z) is calculated by AC Constant Voltage (V) and Current (Im). Capacitors Capacitance value (Z) is calculated by AC Constant Voltage (V), Current (Im) and Frequency (f). Inductors Inductance value (Z) is calculated by AC Constant Voltage (V), Current (Im) and Frequency (f). Guarding Function When loaded boards are measured, occasionally their measured value is not a little far away from the individual value due to influence from the peripheral circuit. So-called “Guarding” is such technique to eliminate such influence as much as possible. Fig. 4 simply explains Guarding principle at DC-CV measurement. While measuring Device Z in Fig. 4, normally measured current Im is divided into two, IRx and IRy. So, it is impossible to measure IRx that flows across the device Z accurately. However, if Point-B could be the same electric potential as Point-G (Guard point), the current IRy is suspended. As the result, IRx is equal with the measuring current Im, so that the device Z is measured accurately. Guarding is not always effective to eliminate the influence from the peripheral circuit. Refer to Undetectable Components in following page. Undetectable Components Table. 1 shows typical samples only, and there are many other unmeasurable cases. To judge whether the component is measurable or not, it is necessary to compare the measured value with the nominal value of the device. 4-Wires Kelvin Measurement “4-Wires Kelvin Measurement” that can eliminate both wire resistance of the measuring cables and contact resistance of the probes for accurate measurement. Fig. 5 simply explains the 4-Wires Kelvin Measurement. If applying the measuring signal, some voltage appears at each R1, Z and R4. Because Current (I) is constant, the current flown across the device Z is not changeable, regardless of R1 and R4 that changes inconsistently. Thus the voltage measurement at the device Z is not influenced at all. R2 and R3 are also connected to the voltmeter that is to measure the device Z. However the internal resistance Rm of the voltmeter is so big (several mega ohms), that it does not influence the voltage measurement at the device Z as well. Accordingly, the device Z can be measured accurately, not influenced by the wire resistance of the measuring cables and contact resistance of the probes. Takaya APT-1600 Element R Mode Range Description DC-CV 5[Ohm]-50[MOhm] Resistor measurement is done while applying the constant voltage (0.1V) DC-CC 0.5[Ohm]500KOhm Resistor measurement is done while applying the constant current(200[nA]-20[mA]) 3Ohm-300KOhm Resistor measurement is done while applying the AC signal (frequency 100[Hz]-100[KHz]). Voltage 0.1[Vpp]. Capacitor measurement is done while applying the constant current (2uA-20mA) AC-100 AC-1K AC-10K AC-100K C L DC-CV 20nF-200mF AC-100 5nF-500uF AC-1K 0.5nF-50uF AC-10K 50pF-5uF AC-100K 0.5pF-500nF ACS-100K 0.5pF-20pF DC-CV 50uH-500H AC-100 5mH-500H AC-1K 0.5mH-50H AC-10K 50uH-5H AC-100K 5uH-500mH Capacitor measurement is done while applying the AC signal (frequency 100Hz-100KHz). Voltage 0.1[Vpp]. Inductor measurement is done while applying the constant voltage (0.1V) Inductor measurement is done while applying the AC signal (frequency 100[Hz]-100[KHz]). Voltage 0.1[Vpp].