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AMC1336: duty cycle on DOUT other than expected

Prodigy 150 points

Replies: 5

Views: 37

Part Number: AMC1336

Hi,

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?

Thank you.