## Low Current Measurement B: Shunt Ammeter

There are two common methods of current measurement – the shunt ammeter and the feedback ammeter. In this post, the shunt ammeter method will be discussed.

1. What are the basic components of a shunt ammeter?

Shunting the input of a voltmeter with a resistor forms a shunt ammeter, as shown in Fig.1. The measured voltage divided by the resistor, yields the measured current.

Fig.1 The shunt ammeter measures the voltage drop across the shunt resistor to calculate the current. (Courtesy: Keithley)

2. Why the shunt resistor should be kept low?

• better time and temperature stability
• better voltage coefficient
• reduce the input time constant (fast instrument response time)
• reduce the burden voltage

3. The shunt resistor should be kept low, however,…. However what?

Thermal noise current increases as the shunt resistor decreases, degrading the signal-to-noise-ratio.

4. Any data of burden voltage from commercial instruments?

Keithley has high performance DMMs. Some data can be seen from one of its product datasheet.

Fig.2 DC current specification of Keithley’s Model 2001 (7-1/2-digits) and Model 2002 (8-1/2-digits) DMM

Let’s make an example. Say an ADC with a 1-V supply and consumes average current around 10nA. The maximum burden voltage is equal to 10nA/200uA * 0.25V = 0.0125mV (the first range is picked). The resulted voltage burden is relatively small compared to 1-V supply.

Obviously, if the test voltage is smaller, the error percentage increases!

5. Besides Keithley’s DMM, any other commercial choice?

Yes, National Instruments. Its PXI-4071 module is a 7-1/2-digits high-performance DMM. Some data extracted from its datasheet:

Fig. 3 DC current specification of NI PXI 4071

* 1ppm = 0.0001%