About small current measurement techniques

In other words, small current is high resistance. There are two basic techniques for measuring small current: shunt method and feedback amperemeter method. In the test, under ideal conditions, the ammeter has no effect on the circuit at all. However, in actual measurement, multiple sources of error may appear. As we discuss below, these sources of error can cause significant measurement uncertainty.

definition of small current
The IC tester is a high-end measurement and will be affected by internal switches, leads, PCB boards, etc., so the current range is generally about 1UA; some special testers for discrete devices such as JUNO machines use low-end measurements and special wiring methods. Can reach NA level. What we are discussing here is to adopt a simple and universal way to realize the test of NA level or below current.

The principle of IV conversion circuit


Must-know, about small current measurement techniques



Must know about small current measurement techniques

Circuit analysis 1: Clean power



Must know about small current measurement techniques

Circuit Analysis 2: Choice of Op Amp

Requirement: The important parameter for testing small current is Ib. If you want to do a micro current test, Ib must be small. Actually. Ib always exists, and it can also be compensated, zeroed, and offset. Of course, it is not as good as Ib is small, because Ib itself is very unstable and will bring current noise, especially its large temperature coefficient, which will interfere with the test results to a large extent. On the other hand, the offset voltage Vos between the positive and negative inputs of the op amp will also affect the accuracy of the test. Vos is directly superimposed on the output signal. Assuming Vos=10mV, then it is originally 1V output, after superposition, there will be 1.01V, resulting in an error of 1%. Assuming that the input current is small, 0.1pA, then the calculated output is only 0.1V, and the actual output is 0.11V, the impact is even greater, reaching 10%. Therefore, Vos is still small. If Vos is not small enough, it can be mostly offset by a compensation circuit. However, Vos has a temperature coefficient, and the output of a temperature change also changes, which makes the temperature coefficient of Vos one of the important indicators.

Circuit analysis 3: R4 feedback resistance

RF is determined by 3 factors:

1, what kind of sensitivity and noise should be achieved;

2, is it easy to buy?

3, what is the IB of the op amp used? IBXRF is a major factor affecting resolution.

Circuit analysis 4: Cf choice

is also 3 factors:

1, offset the input capacitance;

2, affect the time constant, f=2πRC

3, increase the capacity of the op amp to drive the capacitive load;

Circuit Analysis 4: Selection of Capacitors



Must-know, about small current measurement techniques

Circuit Analysis 5: Input Protection

1, a resistor is connected in series at the input terminal of the operational amplifier.

2, add two diodes back to back.

Requirement: extremely low leakage current, appropriate positive current



Must-know, about small current measurement techniques

input protection (increasing leakage)



Must-know, about small current measurement techniques

Circuit analysis 5: Input protection (34401A approach)



Must-know, about small current measurement techniques

resistance noise
The theoretical limitation of small signal measurement lies in the output internal resistance of the signal source.



Must-know, about small current measurement techniques



Must-know, about small current measurement techniques