# Understand attached ckt

Discussion in 'General Electronics Chat' started by aamirali, Oct 26, 2013.

1. ### aamirali Thread Starter Member

Feb 2, 2012
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Title: What is the purpose of pull up/down resistor in attached circuit.

I have adc whose full range will be +-60mv with specified PGA & for thermocouple.

So what is the purpose of pull up here.

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2. ### kubeek AAC Fanatic!

Sep 20, 2005
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they keep the thermocouple in the middle of the supply voltage.

3. ### ErnieM AAC Fanatic!

Apr 24, 2011
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"The two 1-MΩ resistors (RPU and RPD) serve two purposes" and if you read the data sheet to the end you will find the description of these two functions.

4. ### aamirali Thread Starter Member

Feb 2, 2012
415
1
I had read earlier about the function but I don't understand how it helps to keep thermocouple in middle of supply voltage. I had analysed the ckt by designing its equivalent diagram whatever the voltage is at thermocouple, let say 10mv, same voltage will appear on input pins. So what does it mean it keep in middle voltage

5. ### kubeek AAC Fanatic!

Sep 20, 2005
4,687
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Yes, the difference between AIN0 and AIN1 will be for example 10mV, but the both AIN0 and AIN1 will be at about 1.65V with respect to ground.

6. ### aamirali Thread Starter Member

Feb 2, 2012
415
1
but what purpose it solve by keeping both above 1.65V

7. ### kubeek AAC Fanatic!

Sep 20, 2005
4,687
805
It basically makes sure that the inputs are not left floating. The inputs common mode voltage must be between 0 and 3.3V in order for the circuit to work. If you left them without any reference to Vcc or GND they would wander all over the place due to induced noise etc. This way you are sure that the common mode voltage is whre it should be.

8. ### #12 Expert

Nov 30, 2010
16,665
7,310
Keeping the thermocouples in midrange enables the differential amplifier to achieve better common mode noise rejection. Running the thermocouples at either end of the voltage supply converts the differential amplifier into a single ended amplifier with less common mode noise reduction.

(What Ernie said in #3 except I did the reading for you.)