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[AD-FMCADC4-EBZ][AD9680] Full scale input range way too low for AD9680

Question asked by Rubisoft on Feb 8, 2017
Latest reply on Feb 22, 2017 by Rubisoft

Hello,

 

My name is Antony and I'm working on a FMCADC4-EBZ board which includes two AD9680 for a total of four ADC channels. I'm trying to give some sense to read values I get from the AD9680s. For testing, I have plugged the FMCADC4-EBZ board into a Zynq ZC706 with Analog Device HDL running on the FPGA and Linaro Linux running on the ARM CPU also from Analog Device.

 

Using IIO Oscilloscope and/or the IIO C library, I am able to read and write to the SPI registers of my AD9680s, including those relatives to input scale.

 

By default (during boot), Linux sets the input full-scale range (address : 0x25) to 1.58Vpp. But I manually measured that above ~30mVpp, the values I get from the ADCs saturates. I tried to change input range settings using SPI but not even one SPI config ( from Table 10 page 41 of the AD9680 reference manual) could allow me to convert signals above 30/40 mVpp without saturation. I checked SPI communication on other registers and it works fine. I don't think SPI bus is an issue here.

 

Moreover, in Linux, the AD9680 appears as IIO devices. When going to the device folder (/sys/bus/iio/devices/iio:device6/) there is two AXI properties showing up as files : in_voltage_scale and in_voltage_scale_available.

See below:

root@analog:~# cat /sys/bus/iio/devices/iio\:device6/in_voltage_scale
0.076293
root@analog:~# cat /sys/bus/iio/devices/iio\:device6/in_voltage_scale_available
0.073242 0.079345 0.085449 0.091552 0.097656 0.103759 0.109863 0.115966

Four things to notice here.

  1. None of those values may be found in the AD9680 reference manual
  2. Why is the in_voltage_scale default value (the one set during boot) set to a value not listed in   in_voltage_scale_available?
  3. Are those two AXI properties related to the SPI registers responsible for input range?
  4. None of those values explains the ~30mVpp I measured.

If someone succeed to give sense to integers obtained from the AD9680s I would be happy to hear your experience. I don't understand why I'm limited to so low voltages.

Best regards

 

Antony lopez

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