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AD9978 : advices for saturation and precision issues

Question asked by RxC_JBF on Mar 3, 2017
Latest reply on Mar 23, 2017 by RxC_JBF


I'm working on a project with 3 AD9978 working in parallel to make comparative acquisition of a single infrared signal.

The CCD sensors are Toshiba TCD1711DG (linear image sensor).

We already made a first version of the board and even if it is working, we need your help to solve two main problems that are maybe linked :


  1. Saturation

Even with the lowest input gain (-3), there is an input saturation when the infrared source is too close to the sensor. When we observe the output signal of the CCD, it is clear that the peak to peak signal is too high for the 1.4V input range of the ADC.

Is there a specific method to reduce the input signal range to match the AD9978 specification ? Is a resistor bridge dangerous for the signal integrity ?


   2. Precision / jitter

We made repeated measures at constant light source and we observed a very small "jitter" of the measure thought time. Today don't know the origin of this imprecision :

  •  maybe it is due to a wrong sampling time register setting ? but we already made a lot of effort to set it right to the lower part of the curve.
  •  Maybe it is a sync issue between the CCD and the AD9978 ? The clocks are provided by a single CPLD.


This post is just an introduction of our problem, and I am available if you need more details on our system. I will be glad to send you our schematic if you want to check it. Moreover, we will be able to make specific measures with precision oscilloscope with your advice.


Thank you in advance for your time,


Jean-Baptiste Faurie