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AD9826 problem in CDS mode

Question asked by zjq on Apr 11, 2017
Latest reply on May 4, 2017 by zjq

I want to use the AD9826 for sampling a CCD-signal [1-Channel CDS Mode]  (see attachment), regardless of how to amplify the input signal, but the saturation output can not reach 65535.
1. I am using the AD9826 to collect the CCD signal using the CDS mode with an input range of 4V, enabling Internal VREF, using 1-channel VRED input, and Input Clamp Bias setting 4V. The circuit is the datasheet CDS mode using the recommended circuit, AD9826 OFFSET (25 pin ) Parallel to ground 1uF and 0.1uF capacitors.
I am currently collecting signals can not reach full output 65535, the current output of 45000 or so.  please saturation output of about 45000, whether it has a relationship with this, how to improve.

 As shown in Figure 1 CH2 channel (blue line) for AD9826 1 pin CDSCLK1 waveform, CH3 channel (pink line) for AD9826 25 pin OFFSET measurement waveform. That is, when the control AD9826 sampling work AD9826 OFFSET there is a decline, and for the arc-type change 。lease saturation output of about 45000, whether it has a relationship with this, how to improve.

2.As the input signal is enhanced, the offset pin voltage drops more than 3V.

3. The current CDS mode to collect data, data saturation is still around 45000. I added the VINR input waveform, as shown in Figure 3, before the input is 0.1uF capacitor, the input waveform as shown below, with the signal enhancement, the signal will drop to a negative value, what is the reason for this? Can not be saturated output 65535 is related to this.

4. My schematic is shown in Figure 4

5. ADI communication through the telephone, the AD9826 AC grounding test (input capacitor ground). The measurement results are as follows:
    When offset is set to 0 and gain is set to 0 to 6, the output is 0
    When offset is set to -122mV, gain is set to 0 ~ 6, as shown in Figure :

Gain 1

Gain 2

Gain 6