Circuit: A DIY Picoammeter Design

When taking measurements in ionization chambers and other radiation sensors, it's often essential to handle extremely small currents in the picoamp to microamp range. Check out this picoammeter design from 2012.

The Picoammeter Design

The instrument described here can be used to measure currents from around 0.1 pA to 1 μA, without the need to change range. One approach is to exploit the logarithmic characteristic curve of a silicon diode, with the diode voltage buffered by a type TLC272 CMOS opamp. For the first experiment, we used a 1N4148 silicon diode (see l/h circuit). However, the lowest current measurable using this arrangement was over 10 pA, as below that value the diode’s characteristics deviate from the logarithmic curve. 
  A particularly good diode with a very low reverse current is the gate-source diode inside a BF245 JFET (see lower circuit). Using this, we can measure currents of less than 1 pA. To calibrate the circuit, we use known currents at the input and measure the output voltage:
 
The above values closely follow a logarithmic curve: each increase in current by a factor of ten gives an output voltage increment of 70 mV. We have already covered four decades: the graph shows how the curve can be extended to even lower currents.

Experiments show that it is even possible to get useful results with currents of less than 1 pA. In this case, however, it is essential to carefully screen the whole circuit, including the item under test, against the effects of external fields. For the prototype a metal tin was used with feed-throughs.
  Some results obtained with the prototype: 

• ionisation chamber with a sample of pitchblende: about 1 pA;
• BPW34 photodiode used as a radiation detector (in complete darkness): about 10 pA;
• burnt-out filament lamp: about 100 pA;
• burnt-out halogen lamp: about 0.1 pA.

From these last two results, we can see that quartz glass is a considerably better insulator than ordinary glass.

Picoammeter and More

Burkhard Kainka's article, "Picoammeter," appeared in Elektor October 2012, which Elektor Members can download immediately. Member benefits include a subscription to ElektorMag, a 10% Elektor Store voucher for many products, and complete access to Elektor's online library. If you would like a membership, sign up today.

Editor's Note: This article was originally published in a previous edition of our magazine. Please note that some of the products or PCBs mentioned may no longer be available in our store or elsewhere. However, we believe the educational content remains valuable and hope it inspires you to embark on new and exciting projects.