| ??? 03/03/05 10:03 Read: times |
#88955 - Easiest Method Responding to: ???'s previous message |
The easiest method would be to use three fixed resistors (you already know these values because of your manual jumpered scheme) would be to obtain a component called an analogue switch. You simply control the switch via port pins from the processor. Here is an idea of the configuration of a switch from Maxim called a MAX4534 which is a single 4 line to 1 line analogue switch.
http://pdfserv.maxim-ic.com/en/ds/MAX...AX4535.pdf This part's logic inputs are compatible with logic voltage signal levels and yet the power pins can be connected to the same power supplies as your opamp such that the analogue switch can pass signals in the range that would be seen in the opamp circuit. This MAX4534 would be suitable for use with analogue signals and power supplies of up to +/- 20volts. (Do note, if you are unfamiliar with analogue switches, that these things have some series ON resistance and there is some leakage in the OFF state.....nothing is perfect. Also note that almost all analogue switches are limited to handling analogue signals that swing through a range that is INSIDE the range of the applied power supply pins). If the 400 ohm minimum ON resistance of the MAX4534 is too high for your application then go to the Maxim web site and select from one of many other parts. I could comment that you will find that there is a general tradeoff of ON resistance versus the voltage range that the switch can handle. There is a part called a 74HC4052 that you may also consider. These are almost "generic" parts and thus may be readily available in your area. These differ from the Maxim part I recommended above in that the applied analogue signal is limited to a range of about 0 to 10 volts (or +/-5 volts bipolar). A typical 'C4052 has an on resistance in the range of 40 to 100 ohms. Here is a link to the TI data sheet for their 74HC4052. http://rocky.digikey.com/WebLib/Te...,52,53.pdf Yet another part to consider may be the 74HC4066 part. These are even more "generic" than the above part but are strictly limited to unipolar signals from 0v to about 9v depending upon applied power supply and who the manufacturer is. On resistance of these are in the 30 to 100 ohm range. Here is a link to the Toshiba data sheet for their 74HC4066. http://www.toshiba.com/taec/comp...4066DS.pdf Michael Karas |
