AnsweredAssumed Answered

low output amplitude of AD9874

Question asked by czl0047 on Feb 24, 2017
Latest reply on Mar 10, 2017 by czl0047

Hello, I'm using AD9874 to receive signals around 2.65 MHz. The hardware is the AD9874 eval board. I'm using the external clock source and hung mixer mode to prevent LO. The output data format is 24 bits. I have low pass filters after the sampling.

 

My question is the output data is extremely low which is shown in the pic below:

The input signal is already in the "clip point" and I didn't activated the -13dB attenuation in the LNA/Mixer.

 

 

Then I tried to turn on the DVGA, give it a signal with a 2mVpp which led to a weird results (higher signal will led the Q channel to be unstable):

The Q channel itself do have a much more amplitude, but I'm afraid the SNR is still very pool. When I turned off the input, the noise level is around -5 to 5. It 

Please help why does this happen, its best dynamic range was just between 60~70 dB, which is too far away from the 100 dB in the datasheet. I can hardly detect 0.1mV level signal in my application. My configuration script is summarized below:


ADSPIWrite(0x3f,0x99);
ADSPIWrite(0x19,0x8f);//4 wire SPI, 24 bit IQ data
ADSPIWrite(0x00,0x7f);

//LC RC calibration
ADSPIWrite(0x3e,0x47);
ADSPIWrite(0x38,0x01);
ADSPIWrite(0x39,0x0f);

ADSPIWrite(0x1c,0x03);
largeTimeDelay();//10ms time delay
//register 0x1c readbak, if it's not 0, repeat the previous step up to 5 times
ADSPIWrite(0x00,0x74);

ADSPIWrite(0x38,0x00);
ADSPIWrite(0x3e,0x00);


ADSPIWrite(0x00,0x70);

ADSPIWrite(0x07,0xf);//decimation factor = 960
ADSPIWrite(0x1a,0x02);//farthest SSI output frequency
ADSPIWrite(0x18,0x00);

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