analog multiplex question

thermocouple mux

i want to multiplex a 12 bit ADC input.

can i use 74ls373. What is the presion of that multiplexer?

hakan.

multiplexing thermocouples

The number you gave is a TTL latch. I suspect that you made a typing mistake.

In general when evaluating analog switches for this application there are about five important parameters.

One is the frequency response being wider than your signal.

Another is the allowable signal voltage range being greater than your ADC input.

Another is the switching and settling speed.

Another is the on resistance which forms a voltage divider with your source and ADC input impedances. You can calibrate this one out of your measurement system.

Another is the variation of resistance with signal level and temperature.

On way around the resistance items is to have a high impedance ADC input either of itself or with an external buffer. That way the voltage divider effect is 0.9999 or some such number very close to 1 and will not change enough to make 1/4 LSB error with switch temperature and signal level.

multiplexer charge injection

Don't forget to add anti-aliasing filter on each MUX input. Depend on the ADC used, the anti-aliasing filter order should suffice to define the Nyquist bandwidth based on the max frequency of your signal. The MUX selection then shall be made, based on the acceptable settling time due to charge injection and x-talk between the channels. It is strongly reccomended for a high input impedance buffer to be used between the MUX and the ADC, to minimize the effects of MUX dynamic impedance and it's thermal dependency. Naturally proper grounding scheme have to be considered, to minimize the propagation of digital signals into the analog inputs.

4051 multiplexer gnd

i am sorry for mistake. The code is 4051.

and i want to read a thermocouple.

firs i prepared a prototype using a 16f876 and lm358 (non-inverting mode). but i have problem with lsb. there is a unstabil situation. then i decided to use a 12 bit ADC. and i am looking for a low noise opamp. because i belive that i get noise from opamp. but i haven't enough experiance with elektronic. thats why if you have an other idea abouth noise i want to share.

also i want to multiplex input before opamp. because i want to calibrate only one opamp. but input voltage is very low (j type thermo couple, 5.269 mv at 100c) thats why i don't know if multiplexing before opamp is posible or not.

thanks for your kindly helps.

input multiplex

You can reduce the noise by taking a large number of readings and averaging them.

Since your mux is a voltage source, the problems with the resistance of the mux is minimal. The problem you get with thermocouples is the rest of the bimetal contacts. You should have the amplifier and cold junction compensator before any multiplexing.

You can also use a low pass filter to reduce the noise.

thermocouple multiplexing techniques

Here all depends from accuracy you want to achieve. RegUser_2 say well about antialiasing filter and different ground path from analogue and digital. As it was said the impedence is not a problem with thermocouples, because they are a quasi-ideal voltage generators (internal resistence close to zero).

Considering that you want measure temperature from thermocouples you don't need high speed, so you should use a delta-sigma ADC.

Delta-sigma ADCs are oversampled converters, so much less prone to aliasing problems due to noise "shaping" effects.

The market offers a variety of good Sigma-delda with 3 orders modulator, so you can avoid the use of prealiasing filters.

Some of such converters, like AD7730 (Analog Device) have a front-end amplifier with 10/20/40/80 mV Full scale, so very appropriated for direct thermocouples/RTDs measures. AD7730 has two multiplexed channels and a modality of Auto zero and Auto span of the whole measurement chain. Further, 100nA current generators used for continuity check of thermocouples are built-in supplied.

If you want a cheaper but still performant solution, you can select a LTC2418 from Linear Technology. You will have 8 differential inputs and by 24 bits high resolution and possibility to achieve a full range of full scale you can have a direct connection to your sensors.

To create the cold junctions compensator, you may use a single isothermal block and a single thin-film RTD included in middle position of block. You may connect this RTD to a free channel of ADC. Usually the compensating temperature range comes from 5 to 50°C, so in this range the RTD is linear, so you need not to linearize it, still achieving a cold junction error of 0.3°C (best case if you use class A RTD) or a worse 0.5°C. Once you have measured the cold junction temperature from RTD, you must convert this reading in the same equivalent voltage of thermocouple type J at the same temperature and finally you have to add equivalent voltage to FEM measured for that thermocouple. This is because all thermocouples linearization tables are calculated with cold junction to 0°C.
You will use these tables to linearize and scale the thermocouples readings.

anti-aliasing filter and multiplexing

For your guidance, bellow is the block diagram for simultaneus sensor reading :


Sensor1, Sonsor2 ---> SC ----> LPF ---> S/H ---> MUX---> PGA----> C

Description :
Sensor : can be any sensor!
SC : Signal Conditioner that suit to the sensor!
LPF : Low Pass Filter, to make sure for avoiding aliasing
S/H : Sample and Hold circuit controlled by controller
MUX : Multiplexer
PGA : Programmable gain amplifier!
C : Controller, it can be microcontroller or microprossesor!!!!

Hope this can help!