There are 4 ways to classify the electrical signals. Actually, the newest technology covers all kinds of operation mode within one instrument. In this article, we will study 4 kinds of instruments and briefly review the architecture of high performance instrument.
1. Voltage Source / Voltage Measure (VSVM)
If we apply electrical signal on the sample, the response depends on the electrical characteristics of the sample. Using a general power supply, the output of electrical signal is voltage source. This voltage source is designed by several electrical components such as transistor, operational amplifier and so on. The following is simple block diagram of voltage source connected to DUT.
The arrow symbol on voltage source means it is variable. Additionally, voltage source has finite internal resistance Zint. It cause the real output value to wrong, even if we set the value to right one. Ideally, it should be ‘0’ value. In this view, we have to measure the output voltage value on the sample. If a voltage measurement unit(voltmeter) is connected as following diagram, it is possible to implement.
The voltmeter also is made with several electrical components, but it is more common to use instrumentation op-amp. In the past, the total configuration was more complicated due to this various units. The recent developments of circuit technology has made it compact, stable and reliable. Using feedback structure, we need jus one component for VSVM. Later, we will look over this feedback circuit.
However, if the feedback circuit has a problem, it is difficult to know whether the measurement is correct or not. In this case, the instrument will display correct value, but if we use another measurement unit (for example, oscilloscope), real signal may not display the oscillation. If the instrument can measure the current, it is easy to check whether the measurement is ok or not.
2. Voltage Source / Current Measure (VSCM)
It has similar structure as VSVM except to current measurement function. The block diagram is following.
The way of current measurement is indirect way. There is a shunt resistor in series with circuit, and we measure the voltage of the shunt resistance. In this time we have to know the resistance. Then the current can be calculated by Ohm’s law.
The instrument can indicate the wrong value due to a shunt resistor. For example, we assume that the DUT resistance is similar to a shunt resistance. In this case, the half of output voltage value is applied on the shunt resistor, the rest is applied on the DUT. That’s why we choose a shunt resistor as low as possible. For the reference, we can choose few tens or few hundreds miliohm to measure 1 A current. Like this, output impedance is correlated to DUT’s impedance. We need to check the output impedance of the instrument.
3. Current Source / Voltage Measure (CSVM)
When we measure a current, we use a shunt resistor and measure the voltage of the shunt resistor. As same architecture, the current source instrument has indirect circuit structure using voltage control signal. As a result, it has a feedback circuit architecture. To measure a current, it also use a shunt resistor. We can use a textbook for university student to design a constant current source.
Current source has internal resistance. It is differ from a shunt resistor. I is connected in parallel with ideal current source. The block diagram is following.
As a result, a excellent current source has infinite internal resistance. If it is lower then DUT’s resistor, most current will flow in the internal resistor. The shunt resistor can be choose as the previous way.
4. Current Source / Voltage Measure (CSVM)
We can connect a voltmeter to CSCM instrument. It monitor the output status with voltage value. If the current feedback circuit has broken, we can detect it using voltage value. The block diagram is following.
The internal resistance of CSVM should be higher then DUT’s resistance. It is opposite rule to VSCM. We always consider the level of the internal resistance compared to the level of the equivalent resistance of DUT.
As shown before, the electrical instruments are classified as 4 kinds of instrument. However, it is seldom meet these old instrument having simple function in the market. Basically power supply is VSCM, but we can use it as CSVM with current limit function. Most of high performance instruments have all functions in one unit. Additionally, it has sophisticated functions such as over-protection, compliance, average and so on.
* Analog Devices, ‘Data Acquisition’ Homepage Picture
Next time, we will see the signal flow from digital to analog and what is the difference between DC and AC signal on DUT.
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