FET Input Amplifier - temperature problems

[kamaz187]

fet input amplifier

Hi All,

My application involves converting HV AC into low voltage DC which is then fed into an ADC.

I have built a prototype with an Analog Devices AD820 Op-Amp and the circuit is processing the signal correctly at room temperature. However as the junction temperature rises above ~55Deg C the output signal amplitude starts to decrease rapidly with any further temperature rise. I think that this relationship is exponential.

I have attached both the schematics and scope input-output screen.

Has anybody got any idea as to why it would happen and also how to overcome this problem as I need for the gain to vary by a maximum of 1%-2% across the temperatures of 20-100 DegC? (All resistors used are 0.1% tolerance and +- 15PPM/DegC temperature variation)

Thank you for your help!

 

[millwood]

fet input

most chips are rated over -40-80c so 55c shouldn't be a problem.

your issue is likely somewhere else: how did you measure an IC's "junction temperature"?

also, most adcs tend to have very high input impedance so you may not need the opamp.

 

[kamaz187]

amplificatore kamaz

This IC is rated at 85C. That's why I also think that there is a problem with my circuit/ op-amp spec.

The temperature is measured by a temperature IC with I2C interface going back to the same PIC. The temp IC and the op-amp are about 15cm away from each other, but all the equipment is in the same heat chamber with controllable ambient temperature. The actual temperature of the op-amp could be higher than 55C when the problem starts, but it cannot be higher than 75C as this is the ambient temp inside the chamber.

The ADC datasheet says that the input source impedance should be less than 2.5K for the convertion accuracy to be within the spec of +-0.5LSB. I will have to use an op amp because I need the signal to be phase shifted. This is why I used this amplifier in an inverting configuration.

Could this problem happen because of input bias current, offset voltage drift and/or open loop gain variation? What parameters would be most critical in the datasheet?

 

[stefannm]

fet amplifier design

I wouldn't say that this is your problem with temperature, but I would point out that the input impedance of your circuit is around 13.4K. I am sure this is much smaller than the ADC's which will require a much smaller output impedance from the source than just hooking it up directly to the ADC.

What is the benefit of the 15K resistor. Other than setting your input impedance, I am not seeing the value. You could simply remove the 15K resistor and it won't affect the circuit operation (I think).

 

[jorgito]

fet as amplifier

Hello Kamaz,

It surprises me that you say that the op-amp temperature can not go above the test chamber temperature. If I understood correctly, it is almost the same to say that your body temperature must be less than 24 Celsius because you are in a air conditioned room at that temperature.

Regards!

 

[kamaz187]

FET Input Amplifier

I'm sorry. Of course it can, but why would it? It does not draw current from the input and it drives an ADC... I would not expect it to self-heat, unless I've missed something...

Do you think it could be going through a self-heating process???

Thanks for your help.

 

[FvM]

FET Input Amplifier

Which operation shows the said temperature signal drop? The clipped (respectively "rectified") waveform or a small signal frequency response measurement?

If it's the first, how the waveform at high temperature looks like?

If it's the second, what's the bias point and signal amplitude in gain measurement?

What's the "diode" type?