External Power Source with ADC Problem

Created Fri, 02 Nov 2012 09:02:37 +0000 by aflteam

aflteam

Fri, 02 Nov 2012 09:02:37 +0000

Hey,

I have a problem with ADC readings related to External feeding.

My circuit basically is like this. I'm feeding Max32 with 12 Volt on Vin pin. And same 12 V feeds Laser sensor. And from analog output of sensor, i'm reading values with ADC. And printing it out on a basic touch-screen display.

Everything works fine. But there is the problem, when i plugged the USB cable just the Serial Communication for debugging (But 12V still feeding both sensor and Vin of Max32). My touch-screen display shows different values on same operation point. ADC is reading differently when USB plugged in.

External Powering and USB changes somehow ADC results. How can i prevent that? Or what's the cause of this=?

rasmadrak

Fri, 02 Nov 2012 09:22:47 +0000

I can't explain why, I'm afraid - but I've noticed the same. :)

I had a grounding issue when using an external powersupply which went away when I plugged in a USB cable. It seems that the power switches to USB primarily when plugged in - but it still gets juice from the external powersupply if that's plugged in.

Confusious says: With many powers comes great confusion.

aflteam

Fri, 02 Nov 2012 11:45:45 +0000

I can't explain why, I'm afraid - but I've noticed the same. :) I had a grounding issue when using an external powersupply which went away when I plugged in a USB cable. It seems that the power switches to USB primarily when plugged in - but it still gets juice from the external powersupply if that's plugged in. Confusious says: With many powers comes great confusion.

Actually according to the reference design on Page 2 (right below corner) [url]http://www.digilentinc.com/Data/Products/CHIPKIT-MAX32/chipKIT%20Max32_bysa_sch.pdf[/url]

If there is VCC3V3, comparator opamp blocks USB3V3 to interference. But somethings doesn't work as expected , i guess.

aflteam

Tue, 06 Nov 2012 12:11:55 +0000

Well i found something on that matter. I have 2 different max32. Newer one is the one i'm having problem. The old one does not show same error. And i compared the two, they are from different versions.

Does anyone have idea?

GeneApperson

Tue, 20 Nov 2012 18:10:22 +0000

Can you provide a bit more detail about your setup and how the analog readings change? What kind of 12V external supply? How is the power and ground connection made between the power supply, the chipKIT board, and your laser board. How much do the a/d values change? Is it a steady change or does it vary over time?

Can you measure the voltage of the 5V power supply rail when not powering from USB. We had a production problem with one lot of Max32 boards and a 3.3V regulator was loaded instead of a 5V regulator for IC4. Does your new Max32 suffer from this problem?

Thanks, Gene Apperson Digilent

aflteam

Wed, 21 Nov 2012 08:31:15 +0000

Connection Set-up I have 12V 2.1A power supply ( 100-220 AC to 10-13 DC 2.1A Max regulator)

+12 Volt goes to Vin of Max32 and to the power pin of laser. Ground goest to GND of Max32 and to the ground pin of laser.

Laser's output is 4/20 mA, so it connected to adc with 165 ohm resistor. (165x20mA =3.3V)

I had a touch screen to see the results and it's connected via UART1 with Max32.

For example i started the system without usb connection. Screen show 820 (0-1023). But when i plugged the USB and try to see the values from Serial, values goes to 800 and touch-screen shows the same too.

I did not test so much, but change is constant mostly.

I tested the regulator rail. When i supply Max32 with 12V from Vin, 5V on J6 (over 22-23 pins) gives 3.3V.

GeneApperson

Wed, 21 Nov 2012 17:37:31 +0000

Please measure the voltage on both the 3.3V rail and the 5V rail when the USB is not plugged in. Then plug in the USB and see if the voltage of either rails changes.

The A/D converter is using the 3.3V supply as the A/D reference. If the supply voltage changes without the a/d input voltage changing this will cause the converted value to be different.

Gene

aflteam

Thu, 22 Nov 2012 07:32:27 +0000

Well i tested and results should be enlightening.

When 12 Supply is plugged in, 5V and 3.3 rail gives 3.3V only. When only USB is plugged, 5V rail gives 5V and 3.3V rail gives 3.3V.

The situation:

When 12 Supply and USB are plugged in at the same time, 5V rail gives 4.4V and 3.3 V gives 3.3V.

This seems strange. And i noticed that after USB plugged, 12 V supply current goes to Zero.