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| 14.4V ground reference for ADC |
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| 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 \$\Omega\$ 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 :palm:.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?? |
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