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Questions: When ADA4930 driving AD9633

1. The input common-mode voltage range of ADA4930-2 should be 0.3~2.8V. But, i found this parameter has different range in Chinese version datasheet, where is 0.3~1.2V. Also, there is same issue about output voltage range.

2. There are two blocks between ADA4930 and AD9633, which need me to make choices: one is the mach resistor, the DEV kit used 24 ohm, our previous board (between AD8138 and AD9218) used 50 ohm, there is no match resistor in figure 60 of ADA4930 datasheet, however 33ohm in figure 59; aonther is the RLC filter, is this necessary for my application? And  will the inductance destroy the DC coupling fucntion?

 

3.The DEV kit schamatic of the ADA4930 is same as figure 55 of the datasheet, so the ADA4930 was configured as G=1 and input 2Vpp, output also 2Vpp. My system, the front signal is 1Vpp, so i should refer to figure 60, am i right? Which convert the input 1Vpp to differential 2Vpp.

 

4, In dev kit, use Rcm(1.82K) pull-up to +3.3V. There also have several pull-down resistors. Can i remove these pull-down resistor in my design?

 

5. Can you teach me how to calculate each node's voltage based on figure 60 of ADA4930 datasheet? For, datasheet said "For a common-mode voltage of 1 V, each ADA4930-2 output swings between 0.501 V and 1.498 V, providing a 1.994 V p-p differential output. " I can understand how to get 1994 V p-p, but i don't know how to get 0.501 V and 1.498 V.

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  • Hi Coyoo,

    2. The filter will suppress noise, specifically those at higher frequencies. For 80MSPS ADCS, the maximum input you can process is 40 MHz, beyond this frequency the signal is useless and it may add noise and distortion effects to the signal of interest. If you have enough board space, I recommend you to implement a post-amp filter  for noise suppression as well as amp-ADC isolation.

    3. True. You can not implement an Rcm in the tool. But what you can do is play with DiffAmpCalc, find your operating points. Then if you have your target common mode, you can separately implement the Rcm to set the input common mode.

    4. Yes. It is set to 440 mV. To get this value, you need to know the single-ended input impedance. Given in figure 60, Rin_se = 1/[(1/50)-(1/88.5)] = 114.94 ohms. Therefore ADA4930's effective Vicm is 3.3V(114.94/(739 + 114.94)) = 444 mV (440 mV rounded). It's a very complicated process to come up with these resistor values, that's why I highly recommend the DiffAmpCalc tool to compute for those. Adding a pull-up resistor doesn't affect much so it's okay to disregard it in the tool. If you still unclear with the process on how this values are calculated, the application note above would be a great help.

    For the AD8001 question. just create a separate thread for it to be easily tracked down.

    Regards,

    Jino

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  • Hi Coyoo,

    2. The filter will suppress noise, specifically those at higher frequencies. For 80MSPS ADCS, the maximum input you can process is 40 MHz, beyond this frequency the signal is useless and it may add noise and distortion effects to the signal of interest. If you have enough board space, I recommend you to implement a post-amp filter  for noise suppression as well as amp-ADC isolation.

    3. True. You can not implement an Rcm in the tool. But what you can do is play with DiffAmpCalc, find your operating points. Then if you have your target common mode, you can separately implement the Rcm to set the input common mode.

    4. Yes. It is set to 440 mV. To get this value, you need to know the single-ended input impedance. Given in figure 60, Rin_se = 1/[(1/50)-(1/88.5)] = 114.94 ohms. Therefore ADA4930's effective Vicm is 3.3V(114.94/(739 + 114.94)) = 444 mV (440 mV rounded). It's a very complicated process to come up with these resistor values, that's why I highly recommend the DiffAmpCalc tool to compute for those. Adding a pull-up resistor doesn't affect much so it's okay to disregard it in the tool. If you still unclear with the process on how this values are calculated, the application note above would be a great help.

    For the AD8001 question. just create a separate thread for it to be easily tracked down.

    Regards,

    Jino

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