Part Number: AMC1336
i realized a simple circuit to read dc voltage across a resistor.
In the digital part the integrated is powered with a regulated 3.3V and an fpga generates the clock (20MHz) and rereads from dout through a sinc3 filter. DCLK is provided via clock driver with 100 ohms series resistor and DOUT has 33 ohms series resistor.
What I would like to report is that when at the input there is zero volts the duty cycle on dout, measured with an oscilloscope, is not 50% on and 50% off but oscillates continuously doing for example three cycles at 50% and one at 10% then again four cycles at 50% and one at 10% and so on. The duty always oscillates changing input voltage.
However, the result decoded by the internal filter of the fpga, using formulas below, seems consistent with the real voltage value at the input.
bit=1.25/2^23;part=68/(2700*2+68);vread=(word*bit-1.25)/part (where word is the 24 bit word output from sinc3 filter)
could you explain this strange behavior of ic to me?
A picture is sometimes worth a thousand words. Can you provide the scope shots from the oscilloscope?
If the positive input is connected to GND do you still see this behavior?
Alex SmithApplications Engineer | Precision Delta-Sigma Converters
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In reply to Alexander Smith:
thank you for you answer.
below some pictures from the scope.
i tried also to put the positive input to GND but the behavior didn't change.
let me know.
In reply to frantic carlitos:
I've sent an email to one of the designers asking for clarification - but I'm not sure how much I'll be able to share on the public forum.
It is worth noting that these samples are typically averaged in the sigma delta filter module. An oversampling ratio (OSR) of 256 is fairly common. With a clock speed of 21MHz and OSR of 256, your result is an 82 kSPS measurement. Looking at the oscilloscope picture it looks to me like the data output is 50/50 if averaged over a fair number of samples.
ok let me know something about it (what you can share naturally).thanks.
Essentially, it has to do with noise on the inputs in combination with hysteresis within the device.
Something that does concern us is the amount of reflection we see in your scope capture. It is possible that this picture was taken when the scope was not grounded & it is actually cleaner that it appears, but if not we recommend adding some series resistance in the dataline.
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