Comparator input bias current in STM32L073 micro?

[waveboy]

Hi,
Do you believe that the attached LTspice simulation is the best way to analyse the effects of the input bias current in one of the comparators inside the STM32L073 (48 pin) microcontroller?

STM32L073 datasheet
https://www.st.com/en/microcontrollers-microprocessors/stm32l073rz.html

I believe the input bias current is +/-50nA (Pg 102 of datasheet)?

 

[3DDave]

What the simulation says and what the part does are likely to trend the same way, but more depends on what you expect from the simulation result.

Past experience of watching EEs weep over how the parts actually perform vs. how they expect them to perform when running to the edge of datasheet performance says that if you want to know you spin up a breakout board and a scope and watch it yourself.

 

[IRstuff]

Given that a typical datasheet tells you absolutely zero about the specific parameters of the transistors and components used within the circuit, it's unlikely that your simulation is anything other than a vague approximation of the actual circuit.

Even if you knew exactly what the process node the chip was built on doesn't tell you anything about how the transistors are individually constructed; we used to use all sorts of layouts to modify the behavior of transistors to achieve certain circuit characteristics.

 

[waveboy]

Thanks,
BTW, Do you know if this comparator input bias current is more likely to be coming into the comparator's input pin or out of it?

 

[waveboy]

Hi,
The attached comparator circuit takes in some pulsing and turns it into a rail to rail pulsing.
(LTspice and PNG attached)
However, when no pulses come, the signals at the input to the comparator are just 100mV apart.
Also, it is the comparator inside the STM32L073 microcontroller, which has unspecified input bias current,
which may be higher than the 50nA shown here.
Also, the input divider resistances are MegOhm+ and could vary with moisture/dirt ingress.
Would you say this is a reliable design.?
Academically speaking it works, but its shoddy and has no need to be like this.
The circuitry could be easily changed to make the pulsing be rail to rail to the comparator input for example.

Would you go forward with this?....specially since input bias current of the comparator is unknown and may be
significantly higher than the 50nA shown and result in those inputs being even closer in voltage when no pulsing?

 

[IRstuff]

A comparator typically has two inputs, and the bias current is the net difference of the two pins