programmable gain amplifier

Divya,

Below I've posted a screenshot of the single-channel PGAs that TI has to offer which may work for you:

These can be found by going to the Programmable Gain Amplifier section, in the Amplifiers and Linear area on TI.com. Without knowing specifically which gain settings you require for your application, or any other requirements you have, its not possible to narrow down the selection any further. Some information that would help to select an appropriate part are the power supplies required, any package requirements, and signal requirements (bandwidth, settling, non-linearity, offset, etc.). 

Hi,

I am using LM35   to measure the cold junction temperature of thermocouple.The output of LM35 is 10mV/°C and it has to be made proportional to the thermocouple type used,which requires different gain factors(for eg. For J type a gain factor of .005 is required,and this differs for different thermocouples). So I thought of using a PGA at the output of LM35 to get gain factors so as to make the output of LM35 proportional to the thermocouple type used.So is there any PGA available where gain can be programmed  according to my requirements(like gain factor of .005,.004,.0025,.006,.0005)?

Regards,

Divya

Divya,

No PGA that I'm aware of will have gains so far below 1. Perhaps the best approach would be to use a voltage divider to attenuate the output of the LM35 and have different values of resistance in the voltage divider selectable using a multiplexor.  

Hi,

Thanks for the quick response.If I use a voltage divider,what about a type B thermocouple?Type B thermocouple has negative voltage values upto 39°.

Regards,

Divya

You would have the same issue if you used a PGA approach as well, you have to invert the polarity of the compensation voltage below that temperature. Perhaps the most straightfoward approach would be to swap the inputs to your thermocouple amplifier below this temperature. Or you could invert the compensation voltage with an inverting op amp. 

Hi,Divya

What is the temperature range ? what is your cold-junction temperature? and what is your circuit look like? do you use an op-amp or ADC? it helps if you provide more information.If you just use it for linearization,  it doesn't matter what the voltage polarity of the thermocouple.

You can get some information about cold -junction compensation in the below link.

http://www.ti.com/lit/an/snoa663b/snoa663b.pdf

Ragards

Janet Sun

Hi,

Attached herewith is my circuit.The resistor dividers are used to make the output of LM35 proportional to the thermocouple type being used.An analog mux can be used to select among the different voltage dividers.For a type B thermocouple ,how will I design the resistance values for the voltage divider?

8032.ckt_1.doc

Regards,

Divya

Hi, Divya

If you use LM35 to measure the cold junction temperature, I think you can solve the polarity problem as John Caldwell said. At first, I have some misunderstanding about your description. I was thought you use LM35 to do the linearization.

Regards

Janet Sun