MAX1247 4-Channel, 12-Bit ADC

[Atom058]

Hello - I'm back with a question. I was testing my design and code with a power supply that can deliver precise mV outputs in the 0.00 to 102.00 mV range. I was looking to see if my design would work all the way down to 0 mV. Well, I am getting about a 14 mV difference in my readings. In otherwords, my LCD displays 0 mV when 14 mV or below is input. All readings above that are good but off by 14 mV. If I didn't need to measure below 14 mV, I would just add 14 to the value that was being displayed by my LCD and call it good. But I need to see below 14... Anyone have an idea why that might be? And how I might correct it?

Atom058

[Darrel Taylor]

I would tend to believe the problem is with the power supply.

Or more specificaly, the measurement of the power supply's voltage.

A simple test is to use a voltage that you don't need to "read" to know what it is. And the easiest way to do that is with a simple voltage divider.

With 2 resistors 100k to Vdd(+5V) and 220 ohms to Vss. you will get approximately 11mv at the junction. Of course it depends on the tolerance of the resistors you use, but it will still be very close even with 5%.

With 12 bits and 2.5Vref, that should give an A/D value of 22, and due to integer truncating should read 10 mv after the *6/10.

So if it reads 10mv, then the problem is the difference between the actual power supply voltage and the external measuement of that voltage.
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[Atom058]

Darrell - Thanks for your suggestion... Although I was verifying the voltage being output with another meter, I will give it a try (I'll try anything right about now) and let you know what happens.

Atom058