The Floating Differential Source, Part 2: Specifications and Capabilities

The Floating Differential Source (FDS), introduced and explained in the previous article segment, was designed for use in manufacturing test of analog circuits – notably diff-amps in μ C-based data acquisition subsystems. It is short of being a two-port analyzer (TPA) in functional capability and also in manufacturing cost. It saves on instrument expenditure for applications not requiring a full TPA or SMUs. The following specifications define the instrument performance.

FDS Specifications

X dBV = 10X/20

fs: full-scale; zs: zero-scale

Grounded (CM) Source

Output voltage range: +/-0.2 V to +/-35 V

Output voltage resolution ≤ 10 mV

Output current magnitude ≤100 mA

Output impedance, typical: 200 μ Ω || 150 μH

Amplifier closed-loop bandwidth, typical = 6.4 kHz

Amplifier closed-loop voltage gain, typical = 8.8433

DM Source Rejection = Δ VCM/ Δ VDM ≈ 1/10,000 = –80 dBV

Noise ≤ 10 mV rms, 10 kHz bandwidth

Floating (DM) Source

Output +/-fs voltage range: +/-10 V

Output voltage resolution ≤ 2 mV

Output voltage inaccuracy ≤ 250 ppm = 0.025 %

Output current magnitude ≤ 100 mA

Output impedance, typical: 1 mΩ || 1.5 μ H

Amplifier closed-loop bandwidth, typical = 6 kHz

CM Source Rejection = Δ VCM/ Δ VDM ≈ –80 dBV

Noise ≤5 mV rms, 10 kHz bandwidth

Current Source

Output current polarity: port-positive (current sink)

Output current range: 100 mA

Output voltage range (compliance): +5 V to +35 V

Output current resolution = 10 μA

Output current inaccuracy ≤ 250 ppm = 0.025 %

Output impedance, typical: 5 MΩ || 15 pF

CM Source Rejection = Δ IOUT/ Δ VCM ≈ 1/(5 MΩ)

Noise ≤ 25 μ A rms, 10 kHz bandwidth


Input measurement range: +/-4 V

Overrange ≈ +/-25 %

Inaccuracy ≤ 250 ppm = 0.025 %

Measurement resolution = 12,500 counts

Display resolution = 4 decades (4 digit display)

Precision: repeatable to +/-1 count in 4 decades

VCM source display resolution = 10 mV

VDM source display resolution = 1 mV

IOUT source display resolution = 10 μ A

Display type: 7 segment + r.h. decimal-point LED with negative polarity sign LED; 2 LEDs to indicate V or I mode


Voltage mode: OUT+ connected to VDM+ output; OUT– connected to VCM+ output. DM source is in series with CM source and the CM source negative output terminal is grounded.

Current mode: VCM+ is connected to OUT+; OUT– is connected to IOUT.

Serial Port

RS-232-compatible, with three pins used: ground (GND), transmit out (TXD), receive in (RXD); 0.1 inch spaced square pins on edge of circuit-board; solders without cable to pins 1 – 3 of 9-pin RS-232 connector.

Format (factory default): 19.6 kbaud, 8 bits, no parity, 1 stop bit.

Baud rate can be changed by serial-port command, stored in nonvolatile read-only memory (NVROM).

FDS command-line interpreter outputs CR (ASCII 13) when ready for next line. At power-on, header information is output (for reading on a terminal emulator) followed by two consecutive CRs to indicate that the FDS is ready to receive a command character.


Input power: 12 V static (dc) +/-20 %, 1 A

Input power connectors: 2.5 mm power jack, inner conductor positive; 0.25 inch QDC connectors

Power port emitted noise voltage ≤100 mV, 10 MHz

Power port emitted noise current ≤10 mA, 10 MHz


Ambient operating temperature range: 20 o C +/-15 o C (41 o F ,95 o F)

Ambient specified temperature range: 20 o C +/-5 o C (59 o F, 77 o F)

Humidity ≤ 85 %, noncondensing


The grounded and floating sources have medium-performance specifications for a test instrument, placing it within the high-volume mainstream of applications. These capabilities are compatible with testing of many existing IC and semi-discrete amplifiers for differential (Av ) and common-mode (ACM ) gains, CMRR , PSRR , and dynamic input range and resistance, having either voltage or current inputs.

The FDS is of semidiscrete design and the specified performance is achieved using two-point μC calibration. The voltage and current ranges of the sources are suitable for testing the capabilities of typical diff-amps. The power dissipation levels allow for the use of small-signal BJTs in the segmented-amplifier output-stage design of the VCM source. In the FDS implementation, nothing more than TO-92 BJTs are used.

The FDS is an open-source instrument. The FDS Manual is its open-source document, available on request through the Innovatia website. It has enough technical detail that units can be built by anyone desiring to be either an end-user or to manufacture it and become a supplier. (See Planet Analog articles by the author on the open-source designer-supplier electronics industry.) Object-code and board layout (Gerber) files are also available on request to those building FDS units. Limited-edition prototype units are available from the research laboratory of Innovatia at Innovatia. An extension of the FDS is planned that makes it into a two-port or transistor-parameter analyzer (TPA) (curve tracer), the TPA204.

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