LMC662: High temperature opamp (> 200 Deg C) to replace LMC662

Hi Kiran,

have you thought about using forced cooling for the LMC662?

Kai

Hi  Kai,

Thanks for the reply!!!

Yes, I did read the specifications.I'm not sure if 1Gohm is enough for my application. I have a high impedance input.

Is there any method other than forced air cooling to protect the opamp from high temperature like, thermal barrier coating?

Can you suggest any other opamp with high input resistance (>1 T ohm ) and temperature >125 Deg C

Regards

Hi Kiran,

an employee of TI might want to suggest you a suited OPAmp. I only want to point out, that it will be very hard to guarantee an input resistance of >1T at elevated temperatures above 125°C. This has to do with the tendency of input leakage currents and input bias currents to drastically increase with rising temperature.

Kai

Hi  Kai,

Thanks for the reply !!!

Noted. The information you have shared is valuable

Thanks & regards

Paul Grohe,

If I couple the signal to next stage via AC coupling there should be  no worry of error due to input bias current.

Please correct if I am wrong

regards

Hi Kai,

I am building front end electronics for  capacitance transducer which can work in high temperature environment.

it has only 2 amplifier stages.

The first stage is AC coupled to the Second stage and the final output from the second stage is also AC coupled.

regards

Hi Kai,

I am building front end electronics for  capacitance transducer which can work in high temperature environment.

it has only 2 amplifier stages.

The first stage is AC coupled to the Second stage and the final output from the second stage is also AC coupled.

regards

Hi Kiran,

I would not leave the opportunity unused to already obtain some useful signal amplification in the first stage:

To take account of the shaping time constant R1 x C1, choose R3 x C3 >> R1 x C1 and R2 x C2 << R1 x C1. Choose C2 >= 22p to provide sufficient phase lead compensation and R4 >= 47R to introduce efficient isolation from capacitive loads. The DC-gain will be 1 and the AC-gain will be (R2+R3) / R3. R2 and R3 should be chosen low ohmically enough to minimize noise and to increase the overall stability..

Kai

Hi Kai,

Thanks for the advise !!

I had considered transimpedance amplifier for my design but found that the bandwidth will become limitation.

I am looking at a bandwidth of about 150K Hz - 200KHz

regards

kiran

Hi Kai,

Thanks for the advise !!

I had considered transimpedance amplifier for my design but found that the bandwidth will become the  limitation.

I am looking at a bandwidth of about 150K Hz - 200KHz

Also the input is voltage not either  current or charge

regards

kiran

Hi Kai,

Thanks for the reply!!

I  require an opamp with high input impedance (>1T ohm). A typical TIA will have relatively low input impedance

Also the Gain Bandwidth requirements of the opamp will  be high to achieve a bandwidth of 150K Hz to 200KHz

regards

Hi Ronald Michallick,

Thanks for the reply !!

At present I am using a third party sensor and I do not have any electrical model for it

The lower bandwidth limit is not critical in my application. It can be up any value upto 150 Hz

Can I use OPA2333HT in a voltage follower /Buffer  configuration so that I get high input impedance? 

the amplitude of my signal is not exceeding 2 V p-p 

Regards

Kiran 

Kiran,

That op amp has just barely enough bandwidth for your small signal. Slew rate (large signal) is another matter.  For a sine wave , slew rate needed SR = pi*Vpp*F  = 3.14  * 2Vpp * 200kHz = 1.26V/us which is way more than OPA2333 provides. Also max slew rate occurs only when input differential voltage is large which isn't compatible with a virtual ground concept (assuming both inputs are the same)  

Regardless of the op amp chosen, you should test the sensor with input bias current to see how it affect the sensor's operation.

You can do this with a circuit like this or any other circuit that adds current to the sensor. If the sensor can't absorb nano amps of current then you'll have to add resistor to absorb the IIB current. IIB simulated is ("sim voltage" - "U1 out") / 30M. If the sensor easily accepts this small current then the "U1 output voltage wont change much with the emulated bias current. It is also important that the sensor AC output doesn't change with the forced bias current. Keep in mind that just adding the test bias current could affect AC output because the 30M test resistor is also a load to the sensor. 

Hi Ronald Michallick,

Thanks for your advice and interest  in addressing the issue.

1) The sensor output is a 8K Hz pulse. I have considers the harmonics of the 8KHz  pulse to arrive at the bandwidth requirement of 200KHz.

should I not consider f = 8K Hz for calculating the slew rate?

SR = 2πfV

SR = 2*3.14*8000* 2

SR = 0.10 volts/ uS

2)

The sensor I am using has a self capacitance of approximately 100 to 200 pF and is exited by a high voltage DC source of 100V.

Is this circuit arrangement  (30 M ohm resistor) good enough to  absorb the  Bias Current ?

I will also try the circuit you have suggested.  Kindly explain what are resistors and capacitors on the input side meant for in your suggested ckt. (like R1 =10K for providing DC bias to the sensor)

regards

HI Ron Michallick,

I think my slew rate calculation is wrong. Kindly clarify

regards

Kiran

Hi Ronald Michallick,

1) Is the 100V source and R1 symbolic or really added to the circuit.

it is really added to the circuit

2) I have to ask is this sensor is made to work at 210C or hotter?

Yes, the sensor is built to work at 200 Deg C

3) Many sensor manufacturers provide a recommended circuit example showing how to connect the sensor to an amplifier. Is this the case?

No recommendation from the manufacturer

4) What will be the best opamp  to pick with lower Temperature rating (125 C and above)  ? LMP7721 or anything better

Regards

Kiran

Kiran,

With this new information, I can suggest a circuit; I can't suggest a device not rated for the temperature needed.

R3 and C4 help remove the gain peaking that the simulation shows without these components. If the peaking is not troublesome then leave these components unpopulated to save supply current which also reduces self heating of the op amp. The DC shift from input bias current is IB * 30M, so 5nA would be 150mV shift.

OPA2333-HT

OPA2333-HT.TSC

Hi Ron Michallick,

Thanks for suggesting the  scheme.

1) I use  LT Spice for simulating the ckt. ( It is Free). I could not simulate a variable capacitor, hence tried the circuit practically.  For my application  C2 value cannot be less than 2000pF. I arrived at the best combination of C2 R2 trough experiment. Any other value is affecting the shape of the waveform.

2) I am expecting a maximum voltage of 2Vpp at the input of the opamp. Since the error introduced is DC in nature (DC offset) and I'm AC coupling it to the next stage it should not be a problem. 

3) any other parameter to be considered which is critical? please suggest.

3) Though the signal is single ended can I consider using an Instrumentation amplifier. So that I can use a single chip and also get necessary gain ?

Regards

Kiran

Hi Ron Michallick,

thanks for all the help and suggestions 

regards

Kiran

Hi Kiran,

no, this is a snubber to stabilize the OPA2333. You can see the effect when simulating a square wave with steep edges:

kiran1.TSC

Kai

Hi Kiran,

the snubber method is a bit tricky. It may or may not work.

R3 must be as high as the open loop output impedance of OPAmp which is 2k for the OPA2333. So, R3=2k7 is a good choice. After that you try to find a suited C4 to increase the stability. If an OPAmp is instable, a peak in the gain plot in the range of unity gain bandwidth can be seen or a ringing in the step response. See the pictures of my simulation.

Kai

Ron Michallick,

Thanks for the reply

1) what is the type of capacitor I have to select (22nF)

2) what is the capacitor suitable for DC blocking? Please advice (like Polyester etc)

3) How can I accea TINA-TI?

regards

Hi Kiran,

oops, I overlooked your 200°C requirement. These caps can be used:

www2.mouser.com/.../N-5g8m

If they are non-stock, search for another distributor.

For a 200°C application I would prefer the C0G ceramics.

Kai