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Message ID: 2261     Entry time: Tue Dec 4 15:05:56 2018
 Author: anchal Type: DailyProgress Category: TempCtrl Subject: Optimal circuit for transimpedance of temperature sensor for Vacuum Can

## Choosing better TIA opamp (Continued from ATF:2250):

I'm summarizing the analysis of Awade and Zach (40m:8125 and ATF:1752) in the context of the specific application of transimpedance amplifier for the AD590 temperature sensor.

Summary of offsets and thermal coefficients op amps
Parameter \\\\   opamp Unit OP27 OP07 LT1012 OPA827 OPA140 LT1028/LT1128
Input Voltage Offset $\mu V$ 30 30 30 75 30 30
Thermal coefficient of input voltage offset $\mu V/ K$ 0.2 0.3 0.2 0.1 0.35 0.25
Input current offset $nA$ 15 0.9 0.03 0.003 0.005 25
Thermal coefficient of input current offset $pA/K$ 80 8 0.3 Not mentioned Not mentioned Not mentioned
Long term offset drift $\mu V/ Month$ 0.2 0.3 0.3 Not mentioned Not mentioned 0.3
Input bias current $nA$ 20 1.5 0.08 0.003 0.005 40
Thermal coefficient of input bias current $pA/K$ 100 13 0.6 Not mentioned Not mentioned Not mentioned
Voltage noise density $nV/\sqrt{Hz}$ 3.5 @ 10Hz 10.3 @ 10Hz 17 @ 10Hz 7.5 @ 10Hz 8 @ 10 Hz 1 @ 10 Hz
Current noise density $pA/\sqrt{Hz}$ 1.7 @ 10Hz 0.32 @ 10Hz 0.02 @ 10Hz 0.0022  @ 1kHz 0.0008 @ 1kHz 4.7 @ 10 Hz

AD590 datasheet says that it has input current noise density of $40 pA/\sqrt{Hz}$ without mentioning the frequency of this value or corner frequency (or any plot showing this data).

I did a liso analysis with a simple transimpedance circuit of transimpedance = 16667 Ohm and tried all the above opamps. Attached is input referred noise calculated by total output voltage noise density divided by the transfer function of the circuit.

It is clear from this analysis that we would be comfortable below $40 pA/\sqrt{Hz}$ with OP07, LT1012, OPA827, and OPA140. So to choose a particular opamp, we need to look into offset voltage and current drift of these opamps. The voltage offset drifts do not matter much as 1 uV/K translates to 60 uK/K of temperature readout difference per kelvin change in chip temperature. We need to achieve stability up to mK only. The current offset drift of 1pA/K translates as 1uK/K of temperature readout difference per kelvin change in chip temperature. So with given values, in my opinion, even current offset drift does not matter much. But it would be good to have low current offset and bias current features of FET inputs. So the winner is chosen as OPA827.

I'm attaching .fil and notebook file of the analysis in .zip file as well.

Edit: Wed Dec 5 15:39:29 2018

Adding one more calculation here. Assuming that the resistor used is thin film resistor with 50 ppm/K thermal drift, drift in transimpedance would be (1.6k*50e-6) Ohm/K. Then 1 K change in resistor temperature will look like:
$\frac{300 \mu A \times (1.6e3 \times 50e-6 )\Omega}{1 \frac{\mu A}{K} \times 1.6e3 \Omega} = 15 mK$

15 mK change in perceived temperature. So this is the most important factor for us and we need to use the least possible thermal coefficient resistor available with very good insulation of the circuit. On Digikey, I'm seeing a minimum of 10ppm/K resistor for 1.65kOhm value. This should give us 3mK change per K change in resistor temperature. I'll look further if we can get even better resistors.

 Attachment 1: VacCanTemSensorTIAOpampOptimization.pdf  21 kB  Uploaded Tue Dec 4 18:28:56 2018