Simulator test

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Simulator test

Test with OP AMP. The concept of "Virtual Earth" from point "a" is as follows:

Ix = Iref

∴Rx = Vs/ V0*Rref

Vs and rref are excitation signal source and instrument calculation resistance respectively. If V0 is measured, RX can be calculated.

If Rx to be measured is capacitance and inductance, then the v s AC signal source, Rx is impedance form, and C or l can also be calculated.

1.2 guarding

The above test method is for independent devices, but the devices in the actual circuit are connected and interact with each other, so that the IX 笽 ref must be isolated during the test. Isolation is the basic technology of online testing.

In the upper circuit, because of the connection shunt of R1 and R2, the equation of IX 笽 ref, RX = vs / V0 * rref is not valid. In the test, as long as G and F are at the same potential, there is no current flowing through R2, and there is still the equation of IX = Iref, RX remains unchanged. When G point is grounded, isolation can be realized because f point is virtually grounded and the potential of two points is equal. In practice, G and F are equipotential through an isolated operational amplifier. ICT tester can provide many isolation points to eliminate the influence of peripheral circuit on test.

1.2 IC test

For Digital IC, use vector test. Vector test is similar to the measurement of truth table, which excites the input vector, measures the output vector, and judges the quality of the device through the actual logic function test.

For example: Test of NAND gate

For the test of Analog IC, the corresponding output can be measured according to the actual function excitation voltage and current of IC, which is regarded as the function block test.

Non vector test

With the development of modern manufacturing technology and the use of VLSI, it often takes a lot of time to write the vector test program of devices, such as the 80386 test program, which takes nearly half a year for a skilled programmer. With the extensive application of SMT devices, the fault phenomenon of open circuit of device pins becomes more prominent. For this reason, Teradyne has launched the non vector test technology of Xpress, which is introduced by Multiscan; GenRad.

2.1 delta scan analog junction test technology

Deltascan uses the ESD protection or parasitic diodes that almost all the pins of digital devices and most of the pins of mixed signal devices to test the DC current of the independent pin pairs of the tested devices. When the power of a board is cut off, the equivalent circuit of any two pins on the device is shown in the figure below.

1.add a negative voltage to ground on pin a, and the current ia flows through the forward bias diode of pin a. Measure the current ia through pin a.

2 maintain the voltage of pin a, add a higher negative voltage to pin B, and the current IB flows through the forward bias diode of pin B. The current IA is reduced due to the current sharing within the common substrate resistance from pin a and pin B to ground.

3 measure the current ia through pin a again. If ia does not change (delta) when the voltage is applied to pin B, there must be a connection problem.

Deltascan software synthesizes the test results obtained from many possible pin pairs on the device, so as to obtain accurate fault diagnosis. Signal pin, power supply and grounding pin, and substrate all participate in deltascan test, which means that in addition to pin disconnection, deltascan can also detect manufacturing faults such as device missing, reversed insertion, and wire disconnection.

GenRad type tests are called junction Xpress. It also uses the diode characteristics in the IC, but the test is realized by measuring the spectrum characteristics (second harmonic) of the diode.

Deltascan technology is the first technology without additional fixture hardware.

2.2 capacitance coupling test of framescan

Framescan uses capacitance coupling to detect the disconnection of pins. Each device has a capacitive probe on it. When a pin excites the signal, the capacitive probe picks up the signal. As shown in the figure:

1. The multi-channel switch board on the fixture selects the capacitive probe on a device.

2 the analog test board (ATB) in the tester sends the AC signal to each pin in turn.

3. The capacitive probe collects and buffers the AC signal on the pin under test.

4 ATB measures the AC signal picked up by the capacitive probe. If a pin is connected to the circuit board correctly, the signal will be measured; if the pin is disconnected, there will be no signal.

GenRad like technology is called open Xpress. The principle is similar.

This technical fixture needs sensors and other hardware, and the test cost is slightly higher.