STM32F1 ADC Weirdness


That one had me scratching my head for a while.
Depending on the speed of the ADC, there is a small current being eaten by the ADC itself,
in the range of 0.5uA to 3 uA.

Why is that a problem ?

1- It happens even if the ADC is not sampling, which is really weird. When using high current ( a few mA), it is not visible at all.
2- But when using a large resistor (i.e. 300kOhm), that small current is enough to cause a voltage drop of sever tenth of a volt, ruining the measures.
For reference, with the 300 kOhm (or 2x300 kOhm) the current is ~ 10 uA (or 5uA) , so a ADC leaking 2uA is not neglectable at all.

At the fastest speed (worse effect), it causes the capacitor to cap at 3v (or 2.5v) rather than ~3.3v which ruins the estimation.

To confirm, i cut the ADC wire and voila, back to ~ 3.3 v

The estimation of the current is as follows (the different rates/scales of the ADC clock can yield different result for the same sampling period) :


Samp period (us)i (ua)
2.1666666672.966666667
2.8888888892.6
3.4166666672.366666667
4.5555555562
4.52.033333333
61.7
73.333333333
9.3333333331.266666667
210.6333333333
280.5733333333

I really dont understand why.
Oh well, time to put a small op-amp in the middle as a follower and hope it will not cause more problems than it solves.

Let's try first with a LMV324i to avoid messing with 5v and voltage inverter.

Edit : did a quick test with LMV324i, seems to work fine

Comments

  1. DurandAFebruary 23, 2021 at 9:00 AM

    Does the GD32F1 ADC also draw some current? Thanks for your work and posts on the STM32 component tester. I am looking forward to try it!

    ReplyDelete
  2. meanXFebruary 24, 2021 at 12:43 AM

    yes, it is due to the way it works i.e. charge an internal cap , disconnect, then measure that cap.
    That cap is very small ( a few pF) but depending on what you do, it might be a problem if a very small current is a problem.

    ReplyDelete
  3. meanXFebruary 24, 2021 at 12:49 AM

    in other words : GD32F1, GD32F3 and STM32F1 all behave the same, just the internal cap differs a bit

    ReplyDelete

Post a Comment

Popular posts from this blog

Component tester with STM32 : Part 1 ADC, Resistor

Image
I was always  fascinated by the so call "transistor testers" that identify and report pin out, features of pretty much whatever you throw at it. They are based on Atmel 328p chip, the same as arduino nano. So why not do the same thing with a STM32/Bluepill ? The STM32 is faster, has a better ADC accuracy, more memory, can do float etc... and is not more expensive. Warning : I'm doing this for fun & to learn, it might contain error & plain misunderstanding So, i watched a couple of video on the general principle on how they work. This is not a "port" but a rewrite from ~ scratch. When something is weird, i check what the original one does for comparison. Measuring resistance So first thing first :  Resistance & Capacitance. These 2 will unlock the other ones. So how do we measure resistance ? What we do is a resistor divider with a know resistance value: Measure = Vcc*(RtoTest/(RtoTest+Rknown)) As far as the ADC goes, it means ADC=...
Read more

Fixing the INA3221

Image
It 's not too complicated to change the board we have seen  previously . You 'll need to cut some tracks, i did it with a dremel and i'm not very handy, so you'll do better. Red =Cut tracks Blue = Add wire 1- You'll need to separate the bottom connector from the ground, cut the 3 connectors BOTH SIDES. 2- Remove the small red area below the middle resistor to isolate each channels from each other Last, connect right side of each connector to the now isolated pad. Mine looks like this after the operation (ugly i know): Be wary of the small track on the left connector that leads to the left side of channel3 resistor. I cut it while isolating the right pad. Now it works, we can go back to the charger.
Read more

INA3221, weird wiring

Image
Continuing my "smart" battery charger, i've finally received the INA3221 module. It is the purple one you can find on ebay for ~ 5 bucks I soldered a couple of pins to be able to connect it so that i could test it and something immediately caught my attention. It is supposed to be a high side current/voltage sensor, typically in a setup like that : So you can measure the voltage supplied to the load, and the current going through it. Now, let's have a look at the module, look at the top of the following pic, where the sense connectors are. CH3/GND/CH2/GND/CH1/GND ? Wait ? What ? Yes, all the IN- pins are connected to ground, so it is a bottom current only sensor. Additionally, the - side of the shunts are ALSO connected to the ground, no only the connectors. The actual layout is like that : That raises two problems :  The voltage is meaningless as we are only measuring Current x ShuntValue, not the supply volt...
Read more