AD8221 to measure thermocouple with common power pin

Question

Hello, 
 
 i am trying to read a thermocouple which has a shared pin with a heater. 
 The heater is suplied with pwm from another isolated galvanicaly circuit. 
 I have attached a schematic, where you can see how things look like. 
 R5 - is the heater 
 R6 - is the thermocouple 
 V4- is the pwm source for the heater

The gain for the amplifier is set at x100. 
 Of course, in simulation everything works :) 
 But in real life, when i apply power to the heater, the values go crazy. 
 The PWM frequency is 40khz at 50% duty ( 25us period). 
 I read the amplifier output using a 1msps adc in the "off period" of the pwm signal... this gives me a window of roughly 12.5us. 
 What do you think of this aproach? Is it posible to read the voltage accross the thermocouple in this manner. Or do i need to drastically change the design aproach?

Achim · Accepted Answer

Hello Moro, 
 I think you're just following two contradictory strategies. 
 When you filter, you try to suppress the electrical interferences by averaging them out. To improve this you have to take longer timeconstants of the filter and therefore slower reaction times of your signal chain. 
 When you try to sample between the PWM-pulses you go the other direction. You accept, that there is a significant interference during the pulse, but you try to make your signal chain fast enough, so that it can recover from the interference during the PWM-off phase (which should give you a sufficient time window for sampling). 
 I think you cannot follow both directions at the same time, since they have conflicting requirements. 
 Your measurements give valuable information. During the PWM-on phase the differential signal at the amplifier input reaches several hundred mV. Trying to amplify this by a factor of 100 completely overdrives your AD8211 (and maybe also some part of the further signal chain to the ADC).One or several parts of your signal chain leave the area of linear operation and come to a "strange operating condition". 
 When the PWM-pulse is over, tp-tn take just some 10&micro;s to recover. But the overdrive condition in your signal chain takes much longer to recover (many ms). Your filtering with 10kOhm/100nF may contribute significantly to this long recovery time, so you should remove these filter in the first step. But it also could be, that the overdriven signal chain itself takes a time to recover from the strange operating condition in which you have pushed it before. So my recommendation for the next step would be: remove the additional filtering and repeat this measurement. 
 There's an additionial valuable information in the measurements you've shown. When you switch of the PWM, the ADC signal recovers from the interference after ~15ms. But even in the rest of the PWM-pause there ist still another electrical interference from the 50Hz mains. It's visible as a "small" 20ms ripple on the ADC-signal in your first measurement. 
 This is much smaller than the distortion by the PWM. But it is still in the range of ~100mV at the ADC, which corresponds to ~1mV at the thermocouple, so this is still much too high for reasonable thermocouple measurements. 
 Therefore I would again recommend: 
 a) remove your filtering and repeat the measurements to find the PWM-recovery time without your RC filters 
 b) for your final measurement switch the PWM off for the complete recovery time you found in a) plus additional 20ms. During these 20ms without PWM-distortion sample the ADC not only once, but sample it many times and calculate numerically the average ADC-value during these 20ms. 
 This may in the end be good enough for the temperature accuracy you require - or it may not. In the second case, you probably have to go for the fundamental change of your setup. 
 best regards 
 Achim

Achim · Answer

Hello, 
 if your measured values would be exact, the expected ADC-value would indeed be 1250mV - (9mV*100). But have a close look at the accuracy of tp and tn values. If each of them would be off by some mV, a completely different ADC-value would be calculated. 
 In your graph it seems to me, that the resolution of tp and tn is already in the mV-range, so the accuracy-limit of (tp-tn) is for sure several mV. And therefore the measured ADC-value may vary several hundred mV from the value you calculated based on measured tp and tn. 
 A thermovoltage of -9mV would probably correspond to a temperature difference of 100&deg;C ... 200&deg;C over the thermocouple. (Exact value depends on type of your thermocouple). You have the system in front of you, so you may judge best, if this big temperature difference is realistic or if the 9mV were simple wrong (due to inaccuracy of tp, tn measurement). 
 moro said: By the way, the 50hz noise i noticed is coming from my measurement setup ( if i unplug the ac/dc adapter from the laptop the ripple is almost invisible). 
 You have opened the GND-loop by unplugging the ac/dc adapter, and this eliminated the 50Hz interference. If you can ensure, that the GND-loop will never be formed again (e.g. by an end-user, which creates an own GND-connection), you may be able to continue without 50Hz suppression. 
 best regards 
 Achim

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AD8221 to measure thermocouple with common power pin

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