Weird LM358 (clone) behaviour, output rises when difference amplifier goes negative


That one made me scratch my head for some time.

It happens when we use a voltage difference scheme



Image result for opamp difference amplifier

All of a sudden, the ouput raises while the difference is zero or negative

i.e. with all resistors = 1k
When v1=0, and v1 > v2, Vout is raising instead of being zero


WTF ?

It boils down to two lines in the LM358p datasheet :

* "Output  Source : 40 mA"
* "Output Sink : 50 uA with low output"

What does it mean ?

If the OpAmp outputs current, like a simple non inverting amplifier, it will max out at 40 mA.
So with a 1k Resistor, 40V. Not a problem

But, in the case of a difference amplifier, it might want to lower the voltage compared to the "-" pin
In that case, also with a 1k Resistor, it can lower by 50 uA*1k = 50 mV maximum !

It means that if the "-" pin is higher than 50mv compared to the + pin, it will pull the output up

Example:

-Pin=3v
+pin=0v

Output should be zero, it is ~ 3v.

How to fix it ?

Simple, use larger resistors.
The aim os that 50uAxResistor is big compared to the "-" pin.
For example, Instead of 1k, use 20k or 100k.

Comments

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