We are using AD9656 evaluation board and we carry out the following test.
We configure the ADC register correctly with 2,8Vpp input span, quick configuration : One ADC per lane etc.
We put a sinus signal of 1MHz on the input A and check the digital value for different voltage Vpp values.
When we increase the voltage between 100mVpp and 2,5Vpp, from a certain value, we note that the digitalized result is lower than the expected result.We see a compression of data at the end of the curve of data point instead of linear curve.
This is due to the track and hold part of ADC, because we have check the analog front-end and it's work well.
Can we have more information of track and hold structure?What is the value of CPAR and CSAMPLE?We want to make a simulation of this part with LTSpice, but we don't have information of how the ADC take the value of CSAMPLE (discharge of CSAMPLE complete, partial etc.).Where the outputs of the ADC input structure is connected?
Thank you in advance for your answers.
Thank you for using the AD9656.
Figure 46 is a very simplified conceptual diagram. The values of the actual circuit elements are proprietary.
If it helps, the differential input impedance (between…
If it helps, the differential input impedance (between VIN+ and VIN-) is very close to 20Ohms in series with 7pF, in track mode, which is the state of the input right up to the sampling instant.
What sample rate are you using?
Can you share your schematic?
Thank you for your help.
We are using your evaluation board EVAL-AD9656 with the sample frequency set at 125MHz (pdf joined).
We have already implement the track and hold part with the 20 ohms in series with 7pF model in our LTSpice simulation.
During one half period (4ns) the ADC is in tracking mode (Csample is loaded at the voltage value set in the input) and during the second half period (4ns) the ADC is in holding mode (Csample is read by the ADC part system).
But, we don't have any information in the datasheet concerning the status of Csample during holding mode (discharge complete, partial etc.)
What is the value of Csample when the ADC switch from holding mode to tracking mode?
We suspect that these capacitors are not completely charged at the input voltage set when the ADC goes in holding mode.
The ADC input common mode is being applied through 390Ohm resistors. Are you able to measure the input common mode voltage at the ADC inputs? Could you try reducing these VCM resistors to about 100 Ohms to see if that makes any difference?
Your signal generator for both setups is "Keysight 81150A sinus @10MHz/bin 4,688Vpp + elliptic filter 6 order (6dB att @10MHz/bin)".
Thank your for your reply.
In fact, we have checked all polarization voltage at different points of the input structure.The measure result of the input common mode voltage at the ADC inputs is 0,878V with internal ADC Vref equal to 1,392V.
We have already reduce the resistive load in our old design but the dynamic range is lower.Result with 100R and AD8138Fund : -10,67 dBH2 : -83,58 dB --> -72,91 dBcH3 : -86,43 dB --> -75,76 dBcAs you can see the result is lower than before with 390R.We have chosen this value of load (780 R) in order to obtain good performance in term of distorsion. In the datasheet of AD8138 best result with 800R load.
Concerning the other points :
- Fin = Bin x int (Fin_required / Bin) with Bin = Fs/NFs is the sample frequency 125MSPS, N is the number of point 4096 and Fin_required is 10MHz.So, 10MHz/bin mean Fin = 9,979248046875 MHz.
- Ellipic filter is used to concerve only the fundamental signal at Fin and suppress H2 and H3.
- You right. We have not yet calibrate the app and we make only an FFT. For the moment, we want to obtain a good dynamic (close as possible of the datasheet).
I'll ask the amplifier guys if they have any thoughts on your circuit.
One of the amplifier engineers suggested trying to DC couple as an experiment.
If you are able to try DC coupling, replacing the AC coupling capacitors at the amplifier inputs with 0Ohms would be an easy experiment, but you would need to change the voltage on the input shunt resistor center-tap so the signal's common mode voltage matches the amplifier requirements.
Similarly, replacing the capacitors at the amplifier outputs would be easy as well, but in that case you need to change the voltage on the VOCM pins of the amplifiers to 0.9V.
Also, the amplifier engineer said he remembers some types of capacitors did not work well for AC coupling, and contributed to distortion. Do you have any other capacitor types to try?
The amplifier engineer is right concerning the types of capacitors, we have seen also that and we have chosen NPO/COG capacitor.
Before making the changes that you have suggested, we have seen that the VCM level voltage at +/- ADC input is not good (0,74V). So, this cause distortion.
Making your change the dynamic is better and the VCM level voltage at +/- ADC input also (0,86V).Dynamic fund --> H2 : 84,3 dBcDynamic fund --> H3 : 81,9 dBc
But if you seen the datasheet of AD8138 figure 14, we are under the best performance.
ADC input VCM = 0.86V is good.
We need to keep in mind that with multiple elements in the signal chain there will usually be some degradation compared to datasheet performance of an individual component.
How close are you to meeting your acceptable performance levels?
I'll see if the amplifier engineer has any other thoughts.