Two Wire Current vs. Voltage Analyzer adapter
A current vs. voltage plotter or curve tracer is a very common and useful instrument to have around the lab. In addition to its use to characterize the IV characteristics of components like diodes and transistors it can be a useful tool to diagnose and debug circuit faults like shorts due to overstress from static discharge.
The Analog Discovery lab instrument hardware has the necessary differential high impedance inputs and arbitrary waveform generators to be configured into a two wire (or even three wire) current vs. voltage analyzer or curve tracer. The required connections, with the addition of a resistor to sense the current can be wired up on a solder-less breadboard without too much difficulty. But I thought it might be even more convenient to put together a hardwired PC board with a selection of precision current sense resistors and jumpers to configure the possible voltage swing ranges that are possible given the fixed +5V and -5V supplies and the two waveform generators with their +/-5V swings. The schematic in figure 1 shows how these connections might be arranged.
Figure 1 Two wire current vs. voltage analyzer adapter schematic
Differential scope channel C1+ and C1- always measures the voltage across the DUT. Differential scope channel C2+ and C2- always measures the voltage across the current sense resistor.
Jumper 3 selects the current sense resistor value. With no jumper inserted we get the sum of all three resistors, 100Ω + 900Ω + 9kΩ = 10kΩ. With the jumper put in one way we get just the 100Ω and in the other position we get 100Ω + 900Ω = 1kΩ. With these three resistor values we can sense current with scale factors of 10mA/V (100Ω), 1mA/V (1kΩ) and 100uA/V (10kΩ). Other combinations of resistors could also be used I suppose.
Waveform generator W1 always drives the one end of the current sense resistor. The other end of the sense resistor drives one end ( + ) of the DUT. Jumpers 2, 4 and 5 select what drives the other end ( - ) of the DUT. It can be ground, +5V (Vp), -5V (Vn), or waveform generator W2. This allows both positive and negative unipolar (0 to +5, 0 to +10, 0 to -5, 0 to -10) and bipolar (-5 to +5, -10 to +10) voltage swings. The waveform generators (and the fixed supplies) can source and sink up to around 20mA of current.
The built in scope and AWG instruments in the Waveforms software can be used to generate the standard XY I vs. V curves or plots. This would be generally sufficient for curve tracing diodes, LEDs or diode connected transistors. If the second waveform generator W2 were to be used to drive the gate voltage of a FET rather than the negative DUT terminal then this adapter board could also serve as a three wire current vs. voltage analyzer. If a high value resistor were used in series with W2 to drive the base current, a BJT transistor could be analyzed as well. The base current won’t be as precisely known using a simple resistor rather than a true high impedance current source but probably close enough for demonstration purposes.
If one were able to write dedicated software (the user programing interface for Discovery is yet to be published at the time of this writing) then more sophisticated sorts of measurements that require some sort of math could be performed. An RLC meter for measuring unknown resistors or frequency dependent impedance like unknown capacitors and inductors for example. One could even measure the impedance of a loudspeaker vs. frequency with this set-up. Perhaps at some point after the user programing interface is available, you will see some special purpose software for this adapter posted to this blog.
The two sided PC board layout is shown in figure 2. The board is notched out to allow access to the remaining connections (mainly the digital I/O) on Discovery. Note that the female header connector should be mounted on the bottom of the board, so it can plug into the connector on Discovery and the rest of the components go on the top of the board as usual. Simple jumpers are used here but miniature slide switches could be used as well.
Figure 2 Two wire current vs. voltage analyzer adapter layout
A few words about connecting to the DUT. In the layout I’ve included a simple square pin header. This will work well with the female header wires that are supplied standard with the Discovery kit. Wires with alligator clips or micro-grabbers could be soldered to these points as well. Another possibility is to use female banana jacks here so that it uses the same test cables as a handheld DMM.
I’ve attached a zip file containing the Gerber PCB files for the board shown in figure 2 to this blog. As always I welcome the user community out there to post any comments and suggestions to this blog.
BatchPCB.com where I put this board up for public sale has gone out of business. They are directing all their customers to http://www.oshpark.com/
for prototype runs of boards. OSHPark takes Eagle CAD board files directly ( in addition to gerber files ). So I've appended two .zip file containing the Eagle CAD files for a two wire and three wire / DMM interface adapter to this blog.