LPF (low pass Filter) placement.

[ansh1208]

use adc for lp filter

I'm trying to measure temperature from thermocouple. I have amplified the signal(in three stages) and i gave this to ADC but the output value is showing severe jumps. I read somewhere to include LPF (low pass Filter) , should i use this filter just before amplifying the thermocouple voltage or just before ADC input (after all the amplification). & one thing more please, how do i calculate values for the LPF (Sorry i'm basically a mechanical student, but I love electronics, I'm just trying to learn.)

Thanku guys.......

 

[dipnirvana]

placement low pass filter

Please post the circuit so that i may comment. Also, you should put the LPF just before the ADC . I guess you have only DC input to ADC so you may use little slower ADC with required resolution.

 

[python_k]

low pass filter placement

1. maybe a RC network can helpu, R = 50 ohm ~ 100 ohm, C is determined by the signal's cutoff frequency.
2. you can use software filter (like a simple average)to solve this problem.

 

[oermens]

rc lpf adc

if there is no lpf between opamp and adc, what should the output (signal driving adc) be? i have a discrete opamp driving a ADC on a uC and without the LPF it seems there is no output at all, just noise? but when LPF is inserted, the output is correct. is the opamp not able to drive the adc pin without lpf? i know that lpf is for antialiasing, but does it serve any other purpose? is this only caused bc of aliasing?

 

[pauloynski]

lpf just before adc

Your problem seems to be excessive noise comming from the amplifier stage so, please, take care of it. Probably it is due to a large gain and cheap amplifiers combination. So, the output of your amplifier is a signal amplified from the sensor plus noise. Noise has only AC components thus a LP filter can filter it out. You can also use the oversampling technique to filter the signal (provided noise is kept to certain limits) and, probably, using this technique and a simple RC LPF would be enough for your purposes. Please take a look at this http://www.atmel.com/dyn/resources/prod_documents/doc8003.pdf