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Message ID: 2263     Entry time: Mon Dec 10 15:40:51 2018
Author: anchal 
Type: Summary 
Category: TempCtrl 
Subject: Prototype results for transimpedance circuit for Vacuum Can Temperature Sensor 

I soldered a prototype board implementing the transimpedance circuit I, Andrew and Rana discussed earlier.

PFA the schematic and picture of the board.

Some numbers:

Voltage rail: +- 15V
Transimpedance: Rated:16.549 kOhms (15k+1.05k+499)
                             Measured: 16.99 kOhms
Vol Ref; LT 1021DCS8-5 (Actual circuit would have ADR4550ARZ)
Measured voltage at Vol Ref Output: 5.003 V
Measured output at Room Temperature: 8.48 V
Temperature from the measured output: 299.005 K (25.9 ^\circ C)
                                                           It was certainly colder than that. I had no other good temperature reference.
Highest output on waving a hot air gun above circuit: 9.15V
Error in temperature perception from that: 358.5 mK

Possible sources of error:

  • No bypassing capacitors were used. So some voltage noise from power supply must have crept into the circuit.
  • Transimpedance resistor is made up of thin film resistors of 25ppm/K TCR. This translates to approximately 7.5 mK/K (change in perceived temperature per kelvin change in circuit temperature)
  • The resistors in the adder circuit are thick film resistors however their effect should be minimal with even 200ppm/K translating to 1 mK/K.
  • Voltage reference used was LT1021DCS8-5 with 3ppm/K TCR. This translated to 0.8 mK/K, so we should be good with this.

Methods:

For measuring room temperature, I first dabbed the AD590 with little-wet tissue to cool it down. This was done to make sure no remnant heat from previous runs is remaining. Then I put the AD590 inside a shallow metal drawer opposite the soldering bench and waited for some time to let the reading settle. I assumed the metal drawer must be in equilibrium with room temperature and must have high heat capacity to work as a good room temperature bath for AD590.

When seeing the changes due to circuit temperature change, I waved a hot air gun over the circuit so that hot air blows over the circuit and not onto it. The temperature of the hot air gun was 110 ^\circ F with the highest speed setting. I have no exact knowledge of how much circuit temperature I increased but it got pretty hot to touch by the time I reached a ceiling in how much output value is changing (about 9.15 V)

Conclusions:

I think I should go ahead with designing a layout of the circuit. There is only so much we can do in the design of the circuit itself. I have chosen 0.2ppm/K resistors for transimpedance to reduce our major source of drift. For voltage reference, since we are getting more precise ones in the same cost, I chose ADR4550ARZ (BRZ version on Digikey was out of stock) which has lower output voltage noise than LT1021. I have also ordered thin film resistors for the adder stage to completely eliminate the effect of drift from them.

Attachment 1: VCTS_TIA_Prototype.pdf  115 kB  | Hide | Hide all
VCTS_TIA_Prototype.pdf
Attachment 2: 48368130_207018923542708_4747446313996517376_n.jpg  750 kB  | Hide | Hide all
48368130_207018923542708_4747446313996517376_n.jpg
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