Use your serial port for isolated data acquisition

Sept. 15, 1997
Data acquisition using RS-232 or printer ports on the PC has become popular due to easy and non-intrusive connection of serial converter modules. Industrial or medical applications, however, often will require isolation of the grounds from the PC...

Data acquisition using RS-232 or printer ports on the PC has become popular due to easy and non-intrusive connection of serial converter modules. Industrial or medical applications, however, often will require isolation of the grounds from the PC ground for safety and/or noise immunity reasons.

The circuit shown allows for an isolated 12-bit data-acquisition system using the RS-232 port (see the figure). The analog-to-digital converter can be addressed via DTR (data terminal ready), CTS (clear to send), and RTS (request to send), as shown. An isolation transformer is alternately driven with a square wave via the H-bridge-configured quad switch (74HC4066). The low on-resistance of the quad switch allows for less than 80% efficiency at I < 10 mA.

To best utilize the power available from the serial port, the TD (transmit data) also is used to supply current. The TD line is usually at MARK (−12 V). Thus, the polarity of the primary circuit (connected to PC GND) is inverted to take advantage of the current available from TD, which increases the available power by 30%. The two output lines, DTR and CTS, also are inverted and initialized to −12 V (note the inversion of the rectifier diodes D4, D5, and D6).

To operate the output transistor of the U7 optocoupler, a third winding has been added to the transformer along with D3 and C8, thus forming V+. This won’t be necessary if sampling frequencies are under 500 Hz. U2 provides the complementary square-wave outputs while R2, R3, C2, and C3 introduce dead time to prevent shoot-through currents. This circuit can sample at up to 1000 Hz and supply less than 5 mA at 5 V (U6 pin 6) for transducer excitation or other circuitry. Increasing the sample rate beyond 1.5 GHz would starve the optocouplers for current.

The C routine exercises the analog-to-digital converter using COM1, while taking into account the power issues (see the listing). The sample rate depends on how fast the “12 times loop” is executed.

Sponsored Recommendations

Highly Integrated 20A Digital Power Module for High Current Applications

March 20, 2024
Renesas latest power module delivers the highest efficiency (up to 94% peak) and fast time-to-market solution in an extremely small footprint. The RRM12120 is ideal for space...

Empowering Innovation: Your Power Partner for Tomorrow's Challenges

March 20, 2024
Discover how innovation, quality, and reliability are embedded into every aspect of Renesas' power products.

Article: Meeting the challenges of power conversion in e-bikes

March 18, 2024
Managing electrical noise in a compact and lightweight vehicle is a perpetual obstacle

Power modules provide high-efficiency conversion between 400V and 800V systems for electric vehicles

March 18, 2024
Porsche, Hyundai and GMC all are converting 400 – 800V today in very different ways. Learn more about how power modules stack up to these discrete designs.

Comments

To join the conversation, and become an exclusive member of Electronic Design, create an account today!