The Floating Differential Source, Part 1

A few years ago, I was working on the design of a data acquisition system (DAS) that would eventually need to be tested when manufactured. It had both high-side current sensing and floating diff-amp input channels. I could either recommend to the client that some expensive equipment be bought and configured for such testing or else design and build some prototypes of a Floating Differential Source (FDS) which would cost much less. I did the latter. I am now extending the usefulness of the FDS to others as an open-access instrument. (See Innovatia.)

The FDS is an example of an instrument that can measure diff-amp parameters at a fraction of the price of a full SMU or two-port analyzer (multiple SMUs). This two-part article explains what the instrument is. The plans are available on an open-source basis for building your own or in assessing whether to buy one already built.

Instrument Overview

The Floating Differential Source (FDS) is a 6 × 8 inch single-board test instrument, shown below.

It is designed for testing differential amplifiers and high-side current sensing circuits. The analog I/O and circuitry is to the left, the power supply is in the lower-right corner, the AVR 8515 microcomputer ( μ C) is the prominent 40-pin IC at center-right, along with the serial port, and the front-panel (user interface) is in the upper-right corner. No expense was spared in providing an unnecessary enclosure; the four standoffs at the corners of the boards make it a bench-mount instrument. (Four more standoffs could support a simple screen-printed plastic sheet front-panel for those who spill coffee on their workbench.)

The design is in keeping with the Innovatia tradition of minimizing specialized parts and mechanical claptrap. The only customization is in the μ C programming and the two toroidal transductors (magnetic parts) that are hot-glued to the board. The circuit-board itself is the remaining custom part. The μ C can be programmed with a typical digital part programmer such as a Xeltek programmer, given the open-source object code. The toroids are designed to be easy to make by those without power-electronics expertise. If you can use a drill to twist wire into a bundle, count turns, and keep track of winding ends, you can build these parts (the first time) in a couple of hours. The number of different wire sizes is minimized. The cores are readily available and cost about $0.33 US each. The copper wire is somewhat expensive in spool lengths, but you do not need nearly that much for an FDS. The circuit-board is available either as a custom part or as a set of board layout files that can be submitted to a board fabricator. Beyond this, parts acquisition, board assembly, and soldering and testing follow. When finished, what does it do?

FDS Functions

Here is a quick rundown of the subsystems of the FDS:

  • Grounded (common-mode) voltage source with +35 V to –35 V range and ±100 mA maximum output current
  • Floating (differential-mode) voltage source stacked on the grounded source, with a ±10 V, ±100 mA output
  • Grounded current sink of up to 100 mA with a voltage (compliance) range of 5 V to 35 V
  • DVM with four-digit or ±10,000 count full-scale display resolution and ±12,500 count internal resolution
  • Front-panel (user interface): 4 LED 7-segment digits and polarity LED; voltage-mode and current-mode LEDs
  • Serial port: RS-232 communications as alternative command interface to front-panel
  • Power supply: 10-output switching converter, powered from single 12 V dc, 1 A wall supply

The two voltage sources are the grounded source that supplies a common mode voltage (VCM) and the floating source that supplies a differential mode voltage (VDM). The grounded source has an isolated supply, allowing its outputs to be swapped to set the polarity of output voltage when connected to the VCM BNC output and ground. The full-scale (fs) magnitude is 35 V. The VCM output amplifier has a segmented output stage so that TO-92 BJTs can be used over the voltage range.

The floating source has BNC outputs VDM+ and VDM–. It is floating but not isolated from the grounded source because VDM– is (ordinarily) connected to VCM. It provides a differential voltage that is added to the common-mode voltage of the grounded source. However, it can be disconnected and used independent of the grounded source as an input to circuits isolated from the ground of the grounded source.

The grounded current sink sinks current from the IOUT BNC connector and from OUT– in current mode. The OUT+ and OUT– outputs have two configurations: voltage mode (V) and current mode (I), as shown below.

In voltage mode, the floating and grounded sources are stacked in series, as shown. This configuration is useful in testing voltage-input loads such as differential-amplifier common-mode (CM) range, CM rejection, and CM and DM gains and offset, and for determining the gains and offsets for calibrating amplifiers in μ C-based systems.

In current mode, the current sink is connected to OUT– so that current can flow through a current-driven load, such as an input sense resistor, from the VCM grounded source or from VDM+. In this mode, VCM outputs only a positive voltage as a common-mode or high-side voltage. This mode is useful for testing current-input amplifiers such as high-side current sensors or as input to current-measurement channels of a data-acquisition system. By varying the common-mode voltage, the current-sense amplifier input impedance to ground can be measured. Using the current sink, the differential gain, offset, and common-mode gain of the current-input amplifier is measured. Amplifier parameters can also be determined over a range of VCM values by adjusting IOUT to keep the diff-amp output voltage constant. Automated tests can be programmed and executed with serial-port commands.

Now that you know what it does, how well does it do it? In the next segment of this article, we will look at the specifications – that is, the performance capabilities – of the FDS as measured on prototype units. As an open-source instrument, the manual, with circuit diagrams and detailed technical explanation, is available for the asking (from Innovatia), subject to the constraints that you not sell the design (but can distribute it) and that you maintain attribution of the source. Innovatia also has a few limited-edition prototype units available if you do not want to build your own.

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