Printer port activates CMOS switches
Edited by Bill Travis
J Jayapandian, IGCAR, Tamil Nadu, India -- EDN, May 2, 2002
The cost-effective design in Figure 1 provides control for CMOS switches without the need for an external power supply. Analog switches such as those in the MAX4663 are ideal for use in low-distortion applications. They are preferable to electromechanical relays in automatic test equipment or other applications in which you need current switching. The CMOS switches use lower power, consume less board space, and are more reliable than electromechanical relays. The MAX4663 quad switch features 2.5Ω maximum on-resistance, 5-nA maximum leakage current at 85°C, and –56-dB off-state isolation at 1 MHz. They also offer break-before-make switching. The switches operate from a 4.5 to 36V supply or from dual ±4.5 to ±20V supplies.
In Figure 1, the switches mount in and derive power from the PC's LPT port. The design provides as much as 50 mA of current, with current-source compliance as high as 10V. The circuit uses a simple voltage-doubler circuit, the negative-voltage-converter ICL7660, for the separate V+ supply and the VL logic supply. The current source can supply as much as ±200 mA at 10V. Figure 1 shows a current-reversal application in a low-temperature-resistivity experiment. The design draws only a few tens of microamperes from the PC's parallel port. The MAX4663 CMOS switches have complementary pairs (normally open and normally closed). This configuration simplifies the design, with single-enable-bit operation (the switch-enable inputs are shorted together as a single enable). Click here to download a LabView Virtual Instrument program. The program latches the LPT1 port at the address 0x378 with data for forward and reverse operation of the switches. At the LPT port, the D0 and D1 bits (pins 2 and 3) power the circuit in Figure 1. Bit D2 (pin 4) sets the switch-enable/disable function. For data word 0x03, bit D2 is low, enabling the normally open contacts; for data word 0x07, and D2 goes high, enabling the normally closed contacts.
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