14.4V ground reference for ADC

[Arjunan M R]

I am designing a precision bench power supply(18V 1A).I am using an 18 bit ADC (MCP3421) to measure current across a 1 current shunt(I know 18 bit is too precise but it was the cheapest one i can get because its sampling rate was 4.5 Samples / second.But its common mode input voltage range was way too low for me to use it.So i decided to use this circuit so i can use this ADC.What i did is i rather than connecting the GND of the ADC to GND, I used an 14.4V (max. common mode input voltage range was (VDD+0.3) 5.8V. So 18V-14.4V =3.6V)voltage  follower circuit so the max vdd and common mode input voltage range is under the limit of the ADC.And used an isolater (SI8602) to transmit the data to the microcontroller.Will this circuit work?

[Simon]

This sounds over complicated. What you should do as you are in the business of throwing stuff at this just to use this ADC is use a current sensing chip. This can also be done with an opamp but there are lots of nice solutions out there that allow you to measure the voltage drop in a high side sense resistor and get a corresponding voltage out referenced to ground. These also amplify your current sensed voltage allowing you to use milli ohms of sensing resistor. Your 1 ohm resistor will waste 1W when you output 1A, at full 18V that is 5.5% wasted power just in your measurement and as you valtage falls it only get worse. at 5V/1A that is a 20% loss. also it means that your regulator which needs to take feedback from after the resistor will have even more work to do as just pulling more current will cause significant voltage variation in the supply itself

If you use a current sense amp you can have a gain of up to 100 with great precision meaning you only need 10mR and will waste 1/100th the power. At 1A flow you will waste 10mW with a loss of 0.0555% at 18V/1A.

[Arjunan M R]

yeah it might be a good solution but i already ordered this ADC .Please let me know if this circuit will work.

[Simon]

No it won't because your ADC ground will be above the µC one. The problem is not the ADC but how you are trying to use it. It is easier to keep everything referenced to ground. You need to use something like a INA225EVM from Texas Instruments.

[Arjunan M R]

I will use an isolater to transmit data to  the microcontroller.The adc  side  ground will be connected to the 14.4V GND and the microcontroller side  will be connected to real GND.
Will it work??

[Arjunan M R]

I am using Si8602 as an isolater.

[Simon]

Theoretically yes, but it's still a stupid way to do it but go right ahead and make life hard for yourself. It has nothing to do with the ADC, every ADC will have this problem. The name of your problem is "high side current measurement" not "how to float an ADC". How do you think everyone else does it?

[Simon]

where is the positive supply coming from for the ADC, where is the positive reference voltage for the ADC?

[Arjunan M R]

Positive voltage of the adc is the 18V supply for the whole psu.This ADC is internally referenced to 2.048V

[Arjunan M R]

Yes it's stupid to do this.But I am building this psu on a tight budget .So I need to do this.The current sense amp you said is the right way but my online store RS components have it but the minimum quantity is 5.As I said I am on a tight budget.

[Simon]

You are on a tight budget but you are making it expensive for yourself. You are using 2 chips instead of 1. the opamp to create the ground reference and the isolator. Isolators are not cheap but it's your money you are throwing away. Learn this, engineering done to silly budget constarints at best just fails and at worse kills people. You do not have to use the current sense chip I suggested, there are loads of them and many cheaper than the TI one. If you think a programmer can design power electronics and signal processing then think again!

Learn about the building blocks and use them. Don't do the wrong solution just begcause it is the only one you know.

Let me guess, does the voltage regulation happen in the microcontroller by any chance?

[Arjunan M R]

The voltage regulation is done using a DAC which is controlled by a MCU.I will take your words for my next project because I have really done a dumb  thing I already ordered the adc and isolater  don't worry about the opamp it's just an lm358.Thanks for your reply.

[Simon]

That won't work. A µC is too slow to react to load changes plus the conversion and communication time of the ADC. Contrary to what youtube channels like Great Scott say using a microcontroller inside such a critically fast control loop does not work.

You could even use the LM358 to measure you current sense resistor now that you have it but specific chips for this job which are essentially a special purpose opamp cost pence/cents.

So what type of voltage regulator is this? what is doing the power control?

[Simon]

You could also use the LM358 to do the voltage regulation.

[Simon]

You can use digital stuff to monitor the regulator and give YOU feedback but not the main regulation loop.

[Arjunan M R]

Here is the circuit i am using to regulate voltage and controlling the current.Please have a look at it and if you spot any error please notify me.
The dumb thing about this is I don't need that precise 12 bit DAC .But i just used it because it is under my budget.
Because Dave used it in his microsupply(REV A).
So i was sure it will work and I don't haveto respin the pcb.

[Arjunan M R]

  dont mind that i am new to this forum and i haven't  written anything like this in any forum or anywhere in my life.So expect some errors.

[Simon]

where is the schematic from? where is the µC and ADC?

[Arjunan M R]

All schematic I posted are part of the same schematic.I am just posting it like this because if you have to look at something you have to zoom in and the resolution might be too low. ADC MCU DAC LDO are all in the same schematic.

[Simon]

Well there is already a current sense circuit there, you just need to connect your ADC input to that. You have an independant voltage regulator LT3080 so I'm not sure what your trying to do.

[Zero999]

That won't work. A µC is too slow to react to load changes plus the conversion and communication time of the ADC. Contrary to what youtube channels like Great Scott say using a microcontroller inside such a critically fast control loop does not work.

You could even use the LM358 to measure you current sense resistor now that you have it but specific chips for this job which are essentially a special purpose opamp cost pence/cents.

So what type of voltage regulator is this? what is doing the power control?

The problem with the LM358 is its common mode range does not extend to its positive rail.

An op-amp with inputs which do work up to the positive supply can be used with a transistor to amplify and level shift the signal to the 0V rail.

https://www.analog.com/en/analog-dialogue/articles/high-side-current-sensing-wide-dynamic-range.html

[wraper]

That won't work. A µC is too slow to react to load changes plus the conversion and communication time of the ADC. Contrary to what youtube channels like Great Scott say using a microcontroller inside such a critically fast control loop does not work.

It can work and can show great results. However not just general purpose MCU and it will be very tricky.

[Simon]

That won't work. A µC is too slow to react to load changes plus the conversion and communication time of the ADC. Contrary to what youtube channels like Great Scott say using a microcontroller inside such a critically fast control loop does not work.

You could even use the LM358 to measure you current sense resistor now that you have it but specific chips for this job which are essentially a special purpose opamp cost pence/cents.

So what type of voltage regulator is this? what is doing the power control?

The problem with the LM358 is its common mode range does not extend to its positive rail.

An op-amp with inputs which do work up to the positive supply can be used with a transistor to amplify and level shift the signal to the 0V rail.

https://www.analog.com/en/analog-dialogue/articles/high-side-current-sensing-wide-dynamic-range.html

Yea, it can only go to within 1.5V of the positive rail so a voltage divider would be required each side. You loose a small bit of the 1V range but it can be made to work on the budget. there is already a current sense in the circuit that can be used for what the OP wants.

[wraper]

Here is the circuit i am using to regulate voltage and controlling the current.Please have a look at it and if you spot any error please notify me.
The dumb thing about this is I don't need that precise 12 bit DAC .But i just used it because it is under my budget.
Because Dave used it in his microsupply(REV A).
So i was sure it will work and I don't haveto respin the pcb.

It is non functional. For example because all opamps are powered from 5V. EDIT: Nope, not all (point stands) powered from 5V but why in the hell would you note +18 V as VCC?  .

[Simon]

hahaha, i had not noticed. I say again to the OP, if your a programmer start on something simpler......