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This query is related to my old post: ADC outputs random values to constant input

I made further tests, and still struggling. The reason I made a new post is, the query is different. I am using FPGA (Spartan6) to sample the signal. The PMODS can handel a maximum of 3v3. I expected the output of ADC to be below as the input. Also with multimeter I get the same voltage level. But I found in the datasheet (http://www.ti.com/lit/ds/symlink/ads7822.pdf) Page 4 - DIGITAL INPUT / OUTPUT - Logic öevels: Voh = 3V5 (min). That means its from3V5 to 5V. Is my understanding clear?

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  • \$\begingroup\$ Did your previous question get resolved? \$\endgroup\$ – Andy aka May 4 '15 at 15:21
  • \$\begingroup\$ No. Its still open for an answer. I am working out to find the reason and came up with this query. \$\endgroup\$ – dDebug May 4 '15 at 15:22
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If the device is powered from 5V, the digital input levels that will work with the device are between 3.5v and 5V - that is what is said on the table on page 4.

On page 3 it quotes the digital input levels for a 2V7 supply and these are between 2V and 5V.

Please note that the digital output levels will be no higher than the power supply you feed it. If you run from a 3V3 supply then you won't get a logic output higher than 3V3 and probably more like 3V.

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  • \$\begingroup\$ The ADC is powered by 5V and a 3V reference. So as per the datasheet I expect an output between 3V5 and 5V but I always get an output equivalent to the input voltage. This is what I don't understand. If I give a 2V i put I get 2V and I tried all ranges from 0-3V. \$\endgroup\$ – dDebug May 5 '15 at 4:58
  • \$\begingroup\$ I don't understand. Are you saying you get a digital serial stream output level that is peaking at the analogue input voltage? How are you measuring this? \$\endgroup\$ – Andy aka May 5 '15 at 9:06
  • \$\begingroup\$ Measuring with a multimeter. I tried varying the input level with a pot and surprisingly the output varies with the input. It is certainly not expected. \$\endgroup\$ – dDebug May 5 '15 at 9:07
  • \$\begingroup\$ It's a digital serial output and measuring the digital serial stream with a multimeter is not going to tell you anything about the peak output voltage level of the digital stream. If the analogue input is zero volts then the digital stream is going to be all zeros and this will look like 0V on a multimeter. If the analogue input is greater than the ref voltage the serial stream outputted is going to be all 1s and this will look like Vcc. Between these values it may look similar to the input but that is a coincidence based on the number of 1s and 0s in the stream. Use a scope. \$\endgroup\$ – Andy aka May 5 '15 at 9:20
  • \$\begingroup\$ Ok. I understand. Thanks for the detailed explanation. Assuming the output ranges from 3V5 to 5V as per the datasheet, I have one more problem my FPGA can read a maximum of 3V3. Now I have the idea of using a N-MOSFET with a 3V3 input to the drain to limit the maximum voltage. I will get an inverted output. But does this help? Can you give me an idea how do I limit the range to 0-3.3V? \$\endgroup\$ – dDebug May 5 '15 at 9:26

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