AD9645 AD9655 DC cross talk

Hi mizzi 

Thank you for using AD9645/AD9655. We are looking into your query and will get back to you soon.

Hi mizzi ,

Could you provide us with your captures of the DC cross talk, as well as your circuit configuration, for further analysis on our part?

The data sheet does not include DC crosstalk specifications, as the device was evaluated only for AC crosstalk at 70MHz. Testing was performed by driving one channel while leaving the other channel open and observing the resulting AC crosstalk effects on the adjacent channel.

See references for more discussion.
AN-835: Understanding High Speed ADC Testing and Evaluation | Analog Devices
Crosstalkin’ Converters | Analog Devices

Hi JEstrella 

the input stage of out ADC consists of a ADA4930 with 100 Ohm inputs and on both sides a n offset given by a DAC AD5672. 

Vocm is supplied from the ADC. An anti alias filter with 100MHz 18db/octave is attached behind the  the OPamp. 

we have also tested the board with a direct connection of the ADC inputs to external pulse generators (including the correct OCM offset). 

the ADC has the following schematics 

blocking (of AVDD DRVDD and Vref) according to the data sheet with low ESR. 

Various blocking options 100nF 1uF and both to Vref has been tested without any change.  

The input signal on channel1 (100kHz square from an external Pulse generator) to one channel was measured as shown below. 

note:

1.) for an AD9645 we  are using 16bit values  

2.) x-axis is in samples (12500 samples are one period) amplitude ~ 32k LSB (0.5 FS)

The cross talk on channel 0 looks like the signal below!  

cross talk is around 80/32500 = 0.25% and has reverse polarity!  

This is the worst channel of the four ADCs on the board. The cross talk of the other ADCs are around 0.05-0.15%

Note: not all ADC have the same cross talk from channel 0->1 and 1->0  sometimes it changes slightly by 20%

I have also measured cross talk with a 70 MHz (sinus) but with different method as described in AN-835 and this gives quite the same results.

signal channel 1 with signal amp ~ 32k: 

cross talk channel 0 is: 

I do not really know what is described in AN-835! can you provide a more detailed info what was measured for the AD9645?

Thanks for your help 

can you measure the DC cross talk with your test board? 

Thomas 

The AC crosstalk specified in the datasheet refers to adjacent-channel crosstalk, where one channel is driven while the adjacent channel is left open. The crosstalk is determined by observing the spur that appears in the channel left open at the same frequency as the signal applied to the driven channel. The adjacent-channel AC crosstalk was measured using the following two methods:

Method 1: Driving one channel near full scale

• One channel was driven with a signal at approximately -1dBFS
• Leave the other channel open.
• The resulting crosstalk was measured by observing the spur at the same frequency as the applied input signal in the channel that was left open.

Method 2: Driving one channel 3dB above full scale

• One channel was driven with a signal at approximately 3dBFS
• Leave the other channel open
• The resulting crosstalk was measured by observing the spur at the same frequency as the applied input signal in the channel that was left open.

Example:

Note: Crosstalk is measured at 69.5 MHz with -1.0dBFS analog input on one channel and no input on the adjacent channel.

Crosstalk can originate from several sources, including power supply coupling, coupling between signal paths on a PCB, or coupling between channels within the IC itself.

Reference: Crosstalkin’ Converters | Analog Devices

Hello,

With regard to application note, I would take issue with the notion that the "victim channel" be left open when making the measurement.   Instead, the "victim channel" ADC input should maintain the same impedance as the "aggressor channel" ADC for a fair comparison. The reason is that increasing the impedance of the "victim channel" will increase the amount of coupling from an "aggressor channel" that remains at the lower impedance

I would recommend following for debug.
1)  Determine if coupling is from within the ADC or external to the ADC.   
Disconnect differential  outputs of ADA4930-1 and connect series 18 ohm resistors to each other and then tie to VOCMA (after the 10 ohm resistor).   Note that since ADA4930-1 has very low output impedance, shorting the two inputs into differential 3rd order LPF maintains similar impedance.
-Redo measurement and see if any change takes place.

2) Both the ADA4930-1's VOCM output impedance is quite high at 8.4 kohm and is meant to be overdriven from a low source impedance buffer should it need to be set to something different than the (3/10)*(+VS - -VS) nominal level.   The AD9645 datasheet does not provide any information on its VCM voltage output but it is likely high also.   Ideally a low impedance buffer should be driving the amplifiers VOCM input to keep the impedance low so as to maintain proper common-mode voltage of 0.9 V.

How does VOCM_A in the schematic connect to ADC's VCM input?   Ideally the ADC's VCM input should have a 0.1 uF capacitor as close as possible pin as possible 

Note that if a resistor is connecting a VOCM_A to ADC's VCM input...............try removing it and see if it impacts the coupling.   Note that if amplifier is operating at 3.3 V, it should be producing a 1 V  output common-mode which is well within the ADC's input common-mode range.   That said...................a full-scale differential output of 2 Vpp results in 1 Vpp out of each amplifier output such it swings from 0.5 to 1.5 V......which is still within linear operating range of amplfier's output.  

Thank you for help 

we have tried to connect both inputs of the cross talk channel to the Vocm of the ADC as you have described in your sketch ( but we have removed the 18 Ohm resistor to connect the wires). This has  not decreased the cross talk at all.Do you think the 18 Ohm resistor is necessary? 

Also supplying the input of the ADC with a external pulse generator with the correct OCM voltage has not changed the cross talk.  

I have not seen that the OCM input of the AD4930 is also supplying an output voltage with 3/10 of the supply voltage this should be 3.33V*0.3=1V. with our setup we measure 0.9V. thus it seems the ADC is driving the pin correctly.   thanks for that hint. 

the Vocm of the AD4930 is driven directly by the 10 ohm resistor blocked  by C165 (1uF), but we never blocked the Vocm from  the ADC directly at the ADC correctly with 100nF. Thanks for that hint.  

we will try this within of the next days. I will then come back to you! 

If you have a eval board can you connect a square pulse and measure the crosstalk  (10us square length is ok)?  because the eval board is AC coupled you should see an exponential tail pulse, with a high pass of around 1 MHz. nevertheless this should give us a hint for the DC cross talk as measured by us. 

I was wondering about the measurement of JEstrella for the AD9655. the crosstalk yields around -88db (-89dbFS - -1dbFS) . this is around 20db higher than specified in the datasheet (-107db). and very close to the cross talk of some good channels on our ad9655 board.  Any explanation for that?

Thanks

Thomas  

According to the datasheet, a less dense board was used during crosstalk testing to accurately characterize the AD9655's crosstalk performance. 

Hi JEstrella,

You showed a plot from 8/4/26 demonstrating the crosstalk measurement with a crosstalk of -88dbFS.
I assume this was measured using the eval board? The application note AN-835 shows pictures of eval boards. You're saying the crosstalk measurement from the datasheet was taken with a different board? 

Is there any information about the less dense board? 

we can in principle completely isolate the ADC on our board! 

would be nice to get this information! 

Hi, 

we have now checked again our schematics! The Vcm of the ADC was correctly blocked 

The VCM is fed via the 10Ohm resistor and a 1uF blocking C into the AD4930  

Thus the test you described we have done previously without any effect. 

Also we have tried to remove the 10 Ohm resistor and use the 1V internal Vocm from the AD4930. Also without any effect.

Because now no direct connection between the channels is made on the test board and all blocking is made the same way as described in the data sheet i assume the cross talk is given by the ADC itself. 

Especially because the cross talk depends strongly on the temperature of the ADC only, I assume the cross talk is made by the ADC itself and the ADC does not fulfill the specs of the data sheet.  Also the fact that not all ADCs are the same way bad seems to be an indication that some specs are not OK. 

Our board with the AD9655 (same board as for the AD9645, but different chips soldered) behaves the same way but channel which are good on the AD9645 are bad on the AD9655. 

Does anybody know how the cross talk was measured for the measurement of the data sheet? 

"A simplified board was used" is not really helpful. 

Do you have an Eval board? 

Thanks Thomas