I am making a simple circuit that uses the LM335 precision temperature sensor, and the output voltage signal I am receiving from this device has an extremely small range (2.92 - 3.04 V). I want to find an operational amplifier to widen this range to 0 - 5V, but I don't know which op-amp to get. Which op-amp should I buy for my requirements on the LM335?
Hello Tsion,
The output from your LM335 is centered about 2.98 V and would move over the 12 mV over the span you listed. There is the large dc component amongst the temperature-related level change that an amplifier following the LM335 must handle. Then, you need a large dc gain of approximately 417 V/V to amplify the 12 mV change, to a 0 to 5 V change. That 0 to 5 V range then needs to be level shifted to achieve the 0 to 5 V output level you want.
My suggestion is to use a instrumentation amplifier for the amplifier that follows the LM335. The CMOS INA333 instrumentation amplifier would be very suitable for this task. You would need a stable reference voltage of approximately 2.92 V and this would be applied to the inverting input of the INA333. The LM335 output would be applied to the non-inverting input. The INA333 gain would be set to 417 V/V with a ~ 240-Ohm gain-set resistor. The LM337 will amplify the difference between the reference voltage (2.92 V) and the output of the LM335 and amplify it by 417 V/V. However, the resulting 0 to 5 V change has to be centered about 2.5 V to achieve the 0 to 5 V output. The 2.5 V would be applied to the INA333 reference pin. So the circuit requires two accurate reference voltage; 2.92 V and 2.50 V.
No amplifier can swing fully to the power supply levels. The INA333 can swing to within about 50 mV of the power supply rail when driving a 10 k-Ohm load. Increasing the resistive load will allow it to swing even closer. If you increase the power supply to +5.25 V, or +5.5V, then the output will swing all the way to 5.0 V. The low end will still be limited to about 50 mV above 0 V. To truly swing 0 to 5 V a dual supply (+/-15 V) instrument amplifier such as the INA118 would be required.
I hope this helps you get on track with your design.
Regards, Thomas
PA - Linear Applications Engineering
Ok. I am building the circuit on a Dragon-12 USB evaluation board, which has a 5 V power supply that feeds into the circuit. This is for a college project.
So, to summarize what you said: all I need to do is input the resulting voltage from the LM335 into the INA333 through its non-inverting input. Is the LM337 required as well to properly amplify the voltage?
Hi Tsion,
We had some E2E forum problems yesterday and I was able to post my response until now. Sorry for any delay.
The LM337 is a negative voltage regulator and I don’t think you will need it for your circuit. It is best for this application to avoid using a 3-terminal regulator as the reference. Although great for general supply regulator purposes most often they are not precision devices.
I gave your application a little more thought and all that it requires is the 2.92 V reference voltage applied to the INA333 inverting input pin. Then the output of the LM335 would be applied to the INA333 non-inverting input. The 2.5 V reference I mentioned in my previous post is not needed.
About the easiest way to attain an accurate reference voltage is to use a voltage reference IC. I suggest the REF5030 to generate the 2.92 V input level. Although the REF5030 is a 3.0 V reference, it has a Vout adjust pin that allows the actual output voltage level to be adjusted about 15 mV above and below the internal 3.0 V level. Figure 24 in the data sheet shows how this function is utilized. The adjustment will allow you to accurately set the 2.92 V input level and be used to compensate for other offsets in the system.
PA – Linear Applications Engineering