DSO 150: Converting old analog board to new layout ( part 2: ADC Bias)

Now we have to take care of the bias (VBIAS)
For reference, that voltage is used to shift the voltage going to ADC by VCC/2=1.65 v
It is done so that it can sample negative values.
Without it you'd be only seeing positive voltage and all negative voltages would be stuck to ground.

As said previously, the VBIAS was  causing a way for noise & temperature sensitivity to creep in.

The old circuit was powering a 3v Zener from unregulated 8v input and using that as a voltage reference to shift the ADC voltage by ~ 1.65v . Since AV+ was ~ 8v, the R28 resistor was passing 5mA ((8v -3v)/1k), so that the zener was fully on. Now AV+ is only 5v, so the current is only 2mA, which is a bit low.




The new board is using the regulated -5v as voltage reference to bias the last stage of the opAmp, like this :


So the change is simple, remove R15 on the old board and replace it by a 3k resistor connected to -5v (AV-)
The exact value you are looking for is 3030 Ohm,  3k +- 1% is fine.

Since we are touching the bias, you'll absolutely need to recalibrate the DSO afterward.

It is mounted that way : 




BTW, you'll notice that the last stage gain is different between the old and new boards.This is  due to one resistor being different (150 vs 130 Ohm). It's not really a problem.

There is one last optional step to do, get rid of VBIAS completely, that will be for part 3.



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