Hello,
This is the ADC schematic.
I've tested this circuit at -40C.
At - 40C, the ADC value of read by MCU is 5V.
Second picture is about B340A diode forward voltage-forward current graphic
The Operating Temperature Range of diode is -55 / +155.
I think that at -40C, D35 does not forward biased but I dont know why .
But At - 40C, the ADC value of read by MCU is 5V, if so how D36 forward biased?

 

The LM324 has an input bias current of between 45uA and 100uA and this current is in a direction which tends to pull the inputs positive.
This current will pull pin 5 of the LM324 positive and this will reverse bias D36 and so the pin voltage will rise up towards the positve supply voltage and hence the high voltage reading.

 

You're clearly lying. This operating amplifier in this circuit cannot be output at the 5 V output. You need to pass the same current through the diodes to obtain thermal stability.
AlbertHall, The LM324 has an input bias current of between 45nA and 100nA.

 

AlbertHall, The LM324 has an input bias current of between 45nA and 100nA.

You are correct, however this current will still cause the problem I described.

 

You're clearly lying. This operating amplifier in this circuit cannot be output at the 5 V output. You need to pass the same current through the diodes to obtain thermal stability.
AlbertHall, The LM324 has an input bias current of between 45nA and 100nA.

"At - 40C, the ADC value of read by MCU is 5V." I meant approximately 5V, you can think it is like pull up(+5V)

 

The LM324 has an input bias current of between 45uA and 100uA and this current is in a direction which tends to pull the inputs positive.
This current will pull pin 5 of the LM324 positive and this will reverse bias D36 and so the pin voltage will rise up towards the positve supply voltage and hence the high voltage reading.

According to datasheet of diode, it can operate at -55C.
I dont understand the relationship between temperature.
In datasheet of LM324, the input bias current is Temperature Compensated.
Is this problem about bias current?

 

can you please explain what is the purpose of the circuit, and where did you get those 1.5kH (!) inductors to test the circuit with?

 

can you please explain what is the purpose of the circuit, and where did you get those 1.5kH (!) inductors to test the circuit with?

This circuit is about to read engine temperature sender resistance. F6 is a ferrite bead.

 

This circuit is about to read engine temperature sender resistance. F6 is a ferrite bead.

So D35 is the sensing element? Or is it R2?

 

So D35 is the sensing element? Or is it R2?

R2 is sensing element.

 

Is this problem about bias current?

Absolutely yes.

 

R2 is sensing element.

Then why do you have the diodes in there?

 

If the forward voltages of the diodes are supposed to cancel each other then they should have the same current. You can achieve this by connecting a resistor from D36 cathode to gnd with a value equal to R2 plus the DC resistance of F6. This will reduce the current through R2 so you might want ro reduce R8, perhaps 470Ω.

 

See

 

@Bordodynov why do the two diodes in your simulation have different symbols despite having the same label?

 

This is a feature of LTspice's work. When I was drawing the diagram, I just put in a diode. Then I entered the diode selection menu and chose Schottky's diode. The symbol changed. I didn't want to look for the same diode again, so I copied the name of Schottky's diode and put it in place of the name D. If I enter the diode selection menu, the symbol will change to the Schottky diode symbol after closing. In addition, the diode of the shots is also a diode! And there is no error.

 

Note the difference in the value of the high level. My operating amplifier model is made at the transistor level. And as you know, the voltage of the Base Emitter increases when the temperature drops. Therefore, with my model, the output voltage is lower.

 

Then why do you have the diodes in there?

Unfortunately it is an old circuit we are using, in my opinion those diodes should not use for ADC.

 

Note the difference in the value of the high level. My operating amplifier model is made at the transistor level. And as you know, the voltage of the Base Emitter increases when the temperature drops. Therefore, with my model, the output voltage is lower.

I face this problem only when the ambient temperature lower than -20C.

 

I face this problem only when the ambient temperature lower than -20C.

Probably because the leakage of the diodes reduces at lower temeratures. At higher temperatures the bias current may be less than the increased diode leakage.