Multiplexing PT1000 with i.mx28

[XaroRSA]

Howdi peeps.

I am looking to multiplex numerous PT1000's thermometers using a Freescale i.mx28.
I plan to run a single adc channel into the MUX(74HC4067), then out to the sensors.
Do I need to use a input buffer before the MUX? Will the +-80ohms Ron from the MUX serious affect my readings?
The adc max voltage is 1.8V.

Cheers!

[SeanB]

As it is a pt1000 sensor are you using a 2 wire or a 4 wire connection? With 2 wire you will need to cal out lead resistance or accept the temperature error, and for 4 wire you will need 2 switches per channel and buffer them both after the switch to feed the ADC. Best is to arrange any gain setting and filtering on the buffer stage as well, a low pass filter at the input to the amplifier will help, but remember the settling time for the filter after changing inputs to avoid reading the step change as an error.

Use the switches from the same 4067 to switch the wiring from a single sensor, otherwise you will have errors from temperature differences between the IC's affecting readings. Buffer after the IC because the ADC input current will cause issues if you feed the input current through the poorly defined switch resistance. Low pasx filter because temperature is a slow signal. You might also want a low pass filter on each input to the switches, or at least a series resistor for ESD protection, and sample after a delay for settling, or use a series of readings till they settle to a value.

[XaroRSA]

Its a 2-wire connection for all the sensors.
Im using this diagram is a reference:http://openenergymonitor.org/emon/sites/default/files/rtdmodule.png
The i.mx28 has a maximum of 1.8V on the adc channels.
The accuracy is acceptable up to 0.5 degrees, and 1 degree when 100+.
Do you have any examples or links I can check out SeanB?

[SeanB]

No, just fixed a couple of multi input boards on equipment. Most used a buffer after the switch to drive the ADC and do the required gain, filtering and such, and used RC filtering on the inputs along with overvoltage protection ( making it do dual duty as well) to low pass signals in. You could get a stable reading even with a lot of mains nois on the signals.

Are you switching the power to each sensor as well with an analogue switch as well, or just using lots of 1k 0.01% resistors fed from a 2.048V reference via a buffer stage?

[XaroRSA]

Havant decided on the final design yet, but was leaning more to the many  1k 0.01% resistors.