Analog To Digital Converter

There are so many things that can cause any A/D system to report the wrong value. You could be measuring the wrong node. You could have an actual failure of the chip. You may be connected to the correct node but a voltage spike at the moment of conversion dominates the measurement. You may not have an appropriate (inaccurate or wrong frequency) Nyquist anti-aliasing filter. You may not be reading the data from the A/D correctly. You may not be allowing your multiplexer enough time to settle between channel changes. You may have the math wrong and are expecting an incorrect numeric conversion. The most common math error here is forgetting to subtract one for the full scale number. An eight bit A/D has 256 states but the maximum conversion number is 255 not 256. Another common mathematics error comes when the A/D reads a bipolar analog signal. Depending on the digital conversion method (i.e. two's compliment) a variety of different calculation errors can happen. You can even have an analog error from compression, loading, amplifier offsets, and other non-linearities.

Thanks for your reply. what is the solution for all these problems?

Hire an electrical engineer or technician that knows how to measure for each of these plausible conditions and then redesigns your circuitry or software to fix which one is ailing you.

By the way, do you know why a Nyquist filtering network is required for accurate analog to digital conversion?

Hi redfred,

Our OP stated in post #8 that they weren't using a microprocessor with this. Is that even possible? OK, it probably is. Maybe a better question would be: "What would it take to operate this ADC without a microprocessor?" You could probably do it with logic ICs, but with the low cost of 8-bit microprocessors now, why would you want to?

I've downloaded the datasheet, and this product prides itself on being microprocessor-compatible.

Something is very weird here.

As far as operating this chip without a microprocessor, it certainly is possible, but it begs the question why one should put in the extra effort. I can see this being done with a gate array that cycles through a series of known states. Obviously this will be much less flexible than using a microprocessor and the programming of the gate array will make most wish that they had a microprocessor instead. Possibly the OP is splitting hairs and is using a microcontroller (microprocessor with embedded usable memory) instead of a stand alone microprocessor. I'm afraid from the OP reply from my earlier listing of likely errors that the OP is way beyond their grasp.

I'm afraid from the OP reply from my earlier listing of likely errors that the OP is way beyond their grasp.

I'm with you on that one.

Thanks for your reply.

The best thing you could do is show us a graphic of the circuit.

Taking everything redfred said into account, how sure are you of your programming?

Are you sure you're addressing the AD channel of choice correctly?

Are you sure you're addressing the desired AD channel for the right amount of time at the right time?

Hardware is relatively easy (you should recheck this first), but timing is everything!

we are not interfacing with any microcontroller

Then what are you using to tell it what to do?

i connected the output of AD 620 Instrumentation Amplifier with the input of adc 0808

A very good instrumentation amplifier with superb common mode rejection capability. I presume that you realize that your A/D has eight inputs. I hope you've connected to the correct one. But this is not what Mikerho was asking you. If you look at the timing diagram of the A/D itself there is some very specific timing requirements for the digital signals required to get this chip to work properly. Getting these digital pulses from a microprocessor or microcontroller is relatively easy. Question #1 in this game of 20 questions is how do you generate these pulses if you don't have a microprocessor? Additionally, since this is a successive approximation A/D converter it is very important that the analog level does not change more than a LSB during the conversion time.

We would like to help you in this project but if you don't answer our questions we cannot help you.

We externally giving the clock pulse for 1KHz

I think it's hopeless.

Unless the OP shows us a schematic, I don't think we'll ever know what's really going on.

Agreed. It looks like the OP has no understanding of the questions being asked, so they cannot answer them properly. Right now I doubt the the OP knows how to use an oscilloscope to verify chip timing; let alone how an A/D, designed to work with a microprocessor, can work without a microprocessor.

Accurate assessment, I believe. And yes, .

Here are some websites that might help you.

1.) https://forum.allaboutcircuits.com/showthread.php?t=6809

Best of luck and hope the information is helpful. Let us know if you get it working correctly.

BEN

I see that you are new here. Welcome to CR4!

Hi One & All,

If you post the drawing on your circuit with the ADC 0808 in it. I may be able to help you.

BEN