In the last post, shunt ammeter is briefly discussed. In this post, feedback ammeter will be introduced.

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

Fig.1 shows the general configuration of a feedback ammeter. The input current flows through the feedback resistor. The output voltage is a measure of the input current.

2. Compared to the shunt ammeter, what are the advantages of feedback ammeter?

• Lower burden voltage (The low offset current of the amplifier changes the current by a negligible amount.)

• Faster measuring time  (decreased RC due to high gain of amplifier)

3. Any commercial products?

Compared to shunt ammeter, feedback ammeter can achieve better resolution.

Keithley‘s Model 6485 5-1/2 digit Picoammeter can achieve a resolution of 10fA, amazing!

Fig.4 DC current specification of Model 6485 picoammeter from Keithley

Of course, National Instruments also provides a PXI solution. The PXI-4022 guard and current amplifier module implements a feedback ammeter with less than 20 μV of burden voltage on its 100 nA current range. When combined with the NI PXI-4071 DMM (measuring the output voltage), it can possibly achieve precise current measurement sensitivity at 0.5 pA or better.

4. An overall  comparison considering price

Since using NI’s PXI solution, other accessories are needed, which means the price will increase. Based on the comparison, it seems Keithley 6485 is the best choice for efficient low current measurement. However, a suitable instrument is just the beginning of your measurement. Many other error sources during measurement are critical enough to destroy your fragile idea.

God bless every tedious piece of circuit design work!