Analysis of current error caused by detailed explanation of ammeter structure
An ammeter is a device for measure current, and that reading is amperes. There are two kinds of ammeter structures: shunt ammeter and feedback ammeter.
1, shunt and feedback ammeter
Shunt ammeter is a common form and is used in many occasions. Feedback ammeter is more suitable for measuring small current. With the current being smaller and smaller, the application of feedback ammeter is increasing gradually. However, the choice of the correct ammeter depends not only on the magnitude of the measured current, but also on the characteristics (usually impedance) of the device under test (DUT).
2. shunt ammeter: DMMs
Common shunt ammeters are used in almost all digital multimeters (DMMs). The voltage formed by the measured current on the resistance at the input end of the multimeter is proportional to the measured current.
3. Voltage burden
The port voltage of ammeter is called Voltage Borden. As the ammeter is connected in series with the current loop to be measured, the voltage burden will cause the measured current to decrease, so the ammeter cannot accurately reflect the measured current.
An ideal ammeter should have no influence on the current in the current loop, so it needs to have zero input impedance and zero voltage burden. But an actual ammeter will produce a certain voltage burden. Generally speaking, the measurement error caused by the voltage burden of ammeter is equal to the voltage burden divided by the output resistance of the device under test. The voltage burden generated by a typical ammeter is in the order of hundreds of millivolts.
4. Feedback ammeter
Compared with the shunt type, the feedback ammeter is closer to the ideal ammeter, so this ammeter should be selected when measuring the current below microampere level, or when the input impedance is particularly demanding.
