analog multiplexer and rtd interfacing

[solace76]

rtd multiplexer

Hello friends,
I recently designed an 8 channel rtd temperature indicator. I used a common amplifier and philips make 74hc4051's to switch between multiple 3 wire rtd's. Now I have a common current souce and and then an amplifier stage which feeds into a 12 bit adc, what happens is as the ambient temperature changes the Ron of the 4051 change and introduces a error in the temperature display. Is there a hardware/software work around to this problem, I am using a microchip microcontroller 16f72, is there a circuit that I can use to eliminate this or modify the circuit.
I do not want to go in for the analog devices mux/switches as they are very expensive out here.

Best Regards,
Solace76

 

[Kral]

It's difficult to visualize the circuit from your description. By "common current source" do you mean a single current souce that is switched between RTDs? If you buffer the RTD voltages through individual amplifiers and multiplex the amplifier outputs, then the effects of the mulitplexer Ron will be minimized (assuming that the A/D input impedance is high compared with the RTD resistance.
Regards,
Kral

 

[solace76]

hello kral,
waht you say is right, its a single current source for all channel, I dont want to use multiple amps because, the circuit size and cost both will increase, cant somthing be done with the existing circut.

 

[VVV]

First of all, if your current source is truly a current source and it operates properly, then the increase in the on resistance of the switches should not make any difference.
That is provided you sense the RTD voltage AFTER these switches, not before them.
If you can post the schematic or PM me, I can offer suggestions.