AD7779: dynamic parameters

Question

Hi, AD Team! 
 I am measuring the dynamic parameters of AD7779 and I have some questions. 
 1) This is the spectrum I got. I use Pseudo Differential input and follow the InputRange excel guide. The frequency of input sine on AIN+ is 60 Hz, the amplitude is 1,5 V, offset voltage is 1,65 V. Ain- = 1,65 V. 
 Dynamic parameters are worse than those which are presented in datasheet. Which test conditions (frequency, amplitude of input sine) correspond to the parameters in datasheet? It is said that frequency = 60 Hz for SFDR. Is it the same for THD and SINAD? 
 Are there any reasons of such bad parameters? 
 
 2) Also INL is worse than supposed to be. What can be a reason for it? 
 
 3) What are the requirements for jitter? 
 4) What are the requirements for input resistance? 
 5) I use PXI-4110 as a source of external REF. Is it appropriate? Should I use a buffer for it? 
 6) The last question is about SAR ADC: Which input range does it have? When I use AIN+ = 0..3,3, AIN- = 0 I get only a half of its transfer function.

JoanOrtega · Accepted Answer

Hi Maria, 
 I assume you are using unipolar power supply with AVSS tied to GND and AVDD=3.3V. What is your output data rate? 
 
 1) Your FFT looks fine, actually the floor noise is the same as the plots show in the datasheet: 
 
 Since you are using pseudo differential, you are losing dynamic range since you cannot use reference scale so you will not reach 0dB in the FFT fundamental frequency input signal. 
 
 2) the datasheet specifies that INL in differential mode is +-7ppm which translates into +-117LSB if Full scale range is 5V. In pseudodifferential the INL will be worse. 
 3) The jitter of the external clock will influence on the SNR. The datasheet does not specifies any number but you must consider it for you calculations. 
 The following article explains how jitter affects the performance. 
 Sigma Delta ADC Clocking&mdash;More Than Jitter (mouser.com) 
 4) the AD777x makes use of an switching capacitive PGA input stage. Basically any input resistance can be use. 
 5) I do not know this device. we normally use Audio precision converter like AD2700 for my quick measurements but it could be worth to try with and without buffers to see if there is any difference. 
 6) you only see half of the values because you are using just half of the Full scale. Full scale is from -3.3V to 3.3V. The input signal is differential and you are tying Auxmin to 0V so you can not go to negative values. 
 
 Regards 
 Joan)

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AD7779: dynamic parameters