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
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.
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
|| 150 μH
Amplifier closed-loop bandwidth, typical = 6.4 kHz
Amplifier closed-loop voltage gain, typical = 8.8433
DM Source Rejection =
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
Amplifier closed-loop bandwidth, typical = 6 kHz
CM Source Rejection =
VDM ≈ –80 dBV
Noise ≤5 mV rms, 10 kHz bandwidth
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 =
VCM ≈ 1/(5 MΩ)
Noise ≤ 25
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
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.
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 oC +/-15 oC (41 oF ,95 oF)
Ambient specified temperature range: 20 oC +/-5 oC (59 oF, 77 oF)
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.