Design of automatic test equipment for measuring pressure by adding flow and measuring flow by adding pressure

0 Introduction

Automatic test equipment is used to test the DC parameters, AC parameters and functions of discrete devices, integrated circuits and mixed signal circuits. It mainly controls the test equipment units through the test system software to test the device under test to determine whether the device under test meets the device specification requirements.

1 Composition of automatic test equipment

The automatic test equipment is mainly composed of precision measurement unit (PMU), device voltage source (DPS), voltage and current source (VIS), reference voltage source (VS), audio voltage source (AS), audio voltmeter (AVM), time measurement unit (TIMER), relay matrix, system bus control board (BUS), computer interface card (IFC), etc. The system block diagram is shown in Figure 1.

This article mainly introduces the design principle and implementation of the voltage and current source part.

2 Basic principles of voltage and current source

The voltage and current source is an essential part of the automatic test system. It can apply precise constant voltage or constant current to the device under test and can measure its relative current value or voltage value. Therefore, the voltage and current source mainly has the following two working modes:

(1) FVMI (Frequency Measurement by Voltage) method. In the FVMI method, the driving voltage value is provided to the output driver through a digital-to-analog converter ( ADC ); the driving current is sampled by a sampling resistor, converted into a voltage value through a differential amplifier , and then read back by the ADC. The clamp value can be set according to the load. The clamp circuit plays a current limiting protection role here. When the load current exceeds the clamp value, the VIS output becomes a constant current source, and the output current is the clamp current. The test system automatically selects the flow measurement range according to the clamp value.

(2) Current-in-Voltage Measurement (FIMV) method. In the FIMV method, the drive current value is provided to the output driver through a digital-to-analog converter ( DAC ); the voltage is read back by the ADC. The clamp value can be set according to the load. The clamp circuit plays a voltage limiting protection role here. When the load voltage exceeds the clamp value, the VIS output becomes a constant voltage source, and the output voltage is the clamp voltage. The test system automatically selects the pressure measurement range according to the clamp value.

FIG2 is a logic block diagram of a voltage and current source.

[page]

3 Design details of voltage and current source

The basic circuit of the voltage and current source is shown in Figure 3. The left half is the voltage and current adding circuit of the voltage and current source, and the right half is the test circuit.

The circuit consists of a main op amp, a current expansion circuit, a range resistor, a feedback loop and a differential operational amplifier . The circuit is simple and practical. A relay K1 is used to switch between voltage and current. When K1 is open, it is used to apply voltage, and when K1 is closed, it is used to apply current. K8 is a switch for current and pressure measurement. When K8 is open, it is used to measure current while applying voltage, and when K8 is closed, it is used to measure voltage while applying current. The op amp U3 is connected to a subtraction circuit to control the clamping voltage. The current expansion circuit is connected to the main op amp U1, which is composed of a push-pull form to increase the output current of the circuit. The range network is composed of multiple resistors of different levels. The test range can be switched through relays from K2 to K7. The feedback loop is composed of a follower composed of op amps. The test circuit adopts a differential circuit form to improve the test accuracy and reduce the common mode gain of the circuit.

The advantage of this circuit is that it integrates the pressure-adding current measurement circuit and the current-adding pressure measurement circuit very well. Only one relay needs to be switched to realize the switching between pressure-adding current and current-adding, which saves the cumbersomeness of requiring a set of circuits for pressure-adding current measurement and current-adding pressure measurement respectively, and also saves a lot of components.

4 Working Principle

4.1 Test Principle

The following uses FIMV (current addition and pressure measurement) as an example to explain the principle of the circuit. In the FIMV mode, the circuit is simplified as shown in Figure 4.

Since the circuit introduces negative feedback, U1 forms a common-phase summing circuit and U2 forms a voltage follower. Let R1=R2=R3=R4=R.

Since UN1=UP1, from equations (1) and (3), we can get: VIN=UO1-URO. That is, the voltage value applied to both ends of the range resistor is equal to the input voltage value. Since the P2 end of U2 is virtually disconnected, most of the current flowing through RO flows into RL, so the circuit can provide a stable current. As long as the voltage at the URO end is tested, the voltage of the load under the applied current can be tested, thereby realizing current addition and voltage measurement.

Measuring flow by pressurization is similar to measuring pressure by flow, so we will not go into details here.

4.2 Implementation of clamping

This circuit can perform voltage or current limiting protection through the clamping voltage or current value set by the program. When the voltage or current detected by the circuit exceeds the set value, the circuit will be self-protected. The self-protection process is as follows: When measuring voltage by adding current, control the input value of the clamping DAC and change the range resistance, set the output clamping voltage, and then switch the current adding mode to the pressure adding mode, and the measured voltage value returns to the clamping voltage value. When measuring current by adding pressure, switch the range resistance to the maximum range, switch the pressure adding mode to the current adding mode, and output the set clamping current by controlling the input value of the clamping DAC and changing the range resistance, and the measured voltage flow returns to the clamping current value.

4.3 Range calculation

Testing different voltage or current values ​​requires different range values ​​to ensure the accuracy of the test results. Therefore, it is necessary to select the range before testing, and the range selection can be achieved through the program.

The calculation formula for range selection when measuring flow under pressure is:

In the formula, MAX_V is the maximum output voltage of the main op amp, FV is the applied voltage value, Ri is the set clamping current value, and RF is the range resistance. The actual Rf value is the next level of the calculated Rf (for example, if the calculated Rf = 1.6k, the actual R should be 1k (if the next level is 1k).

The calculation formula for range selection when adding flow and measuring pressure is:

In the formula, MAX_V is the maximum output voltage of the main op amp, Rv is the set clamping voltage value, Fi is the current value applied, and Rf is the range resistance. The actual Rf value is the next level below the calculated Rf.

5 Circuit Improvement

In this circuit, the output end is directly connected to the load. When testing small resistance, test errors will occur due to line loss. Therefore, in actual design, the output end and the load can be connected using the Kelvin bridge method, and the two ends of the load are connected using four wires (the two ends of the circuit output are each connected to the load with a pair of "applied wires" and "sensing wires").

6 Experimental Results

In actual testing, the op amp can be a high-precision op amp with low offset voltage and low temperature drift, such as OP07. The sampling resistor uses a high-precision resistor with low temperature drift of 2ppm and 0.01%. The op amp power supply uses a dual 24V power supply, and the current expansion circuit uses a dual 36V power supply. The voltage and current source can be pressed from 0V to 21V, and the current range is from 0mA to 200mA. The accuracy of voltage measurement and current measurement is greater than 0.1% (both DAC and ADC are 16-bit data bits). The experimental results show that the voltage and current source has fast application and measurement speed and high accuracy, and is suitable for rapid detection of integrated circuit parameters.