A Propeller-Based Picoammeter/Microvoltmeter
Photodiodes and photomultiplier tubes are used to detect light; when they are illuminated these devices produce small photocurrents. Imagine how convenient it would be to directly connect these detectors to a complete current measurement system, with a data logging capability. That is the primary purpose of this project.
The traditional method for measurement of small currents utilizes an operational amplifier (with a low input offset current) configured as a current amplifier, or trans-impedance amplifier. In this configuration, a high value feedback resistor Rf is placed between the op-amp’s inverting input and its output pin. An input current is then converted into an output voltage according to
Vout = -IinRf
This output voltage could serve as the input to an analog-to-digital converter (ADC), so as to convert the input current into a digital value.
For measurement of really tiny currents, the Texas Instruments DDC112 chip is a very attractive option. This IC allows charge to be integrated for a precisely determined period of time to compute a current, and an on-board 20 bit ADC digitizes the result. Charge can be measured continuously from two channels; while one pair of integrators is measuring, the accumulated charge from a second pair of integrators is being read out. There are two key advantages of this approach : a high ohmic value resistor Rf (that is expensive) is not needed and the separate current-to-voltage conversion and analog-to-digital converter steps now become one - simplicity itself.
Interaction with the instrumentation is via a LabVIEWTM vi, providing a convenient graphical user interface to both control the DDC112 operating mode and acquire data. The extremely low current noise of the system allows it to resolve currents that differ by less than a few hundred fA when measuring currents in the pA-nA range. This makes the instrumentation ideally suited for low light level detection using directly connected optical detectors such as photodiodes and photomultipliers. LED’s can even be directly connected and used as wavelength-selective detectors (see the discussion below).
The PCB also has a 24 bit ADC on-board (an LTC2440) for measurement of signals ranging from microvolts to volts.
In this case the DDC112 input pins are left unconnected, in order to probe the inherent input noise of the system. After 40 seconds of data sampling the offset current is reported as -1.7 pA with a standard deviation of 270 fA.
To achieve this level of performance the PCB housing the electronics must be placed underneath a grounded metal enclosure.
The current measured by the instrument is 0-3 nA, as expected, with excellent linearity noted.
Photodiode Dark Current Measurement
In the next experiment, the dark current of a HP4220 photodiode is measured over a 40 second period to be ~ 8.8 pA, with a standard deviation of just 1.8 pA. The yellow trace shows a histogram of the current measurement while the green trace is the current vs time trace.
Using LED’s as Optical Detectors
Many years ago, Forrest Mims reported on the use of LED’s as light detectors. When light of an appropriate wavelength strikes an LED, a small photocurrent is produced.
Although not as sensitive as photodiodes, LED’s do have a distinct advantage - they only detect light in a fairly narrow band of wavelengths typically shifted slightly to the blue (i.e. to shorter wavelengths) when compared to that same LED’s corresponding emission spectrum. This means that while a violet LED will respond to violet light, it will not respond to green, yellow or red light.
The data shown opposite shows the dark response of a turquoise green LED measured with the DDC112. Here, the mean dark current is ~ 5 pA with a noise SD of 3 pA.
At the ~ 12 and 25 second marks, the LED is exposed to light emitted by a blue LED for a couple of seconds. With the blue LED approximately 30 cm from the detector a current of 12 nA was measured (peaks are clipped on the scale used here).
The LED showed no response to a high brightness red LED positioned directly in front of it.
Picoammeter Shield for XMOS Startkit
This shield plugs into the J7/J8 headers on an XMOS Startkit. It is functionally identical to the Propeller-based design described above, using the DDC112 charge converter chip. The vacant pads at the left of this PCB are intended for use in creating a bipolar input range for the DDC112, as described in the article by Jim Todsen at the TI website. This capability will allow the PCB to measure currents produced by a photomultiplier tube.