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THS4513-SP interface to ADS5463-SP
TI had suggested using the THS4513 (or THS4511) amp to drive an ADC. Our ADC selection is the TI ADS5463-SP. On page 17 of the ADS5463-SP data sheet an example circuit for using the THS4509 to drive the ADC is shown in figure 17.
Can the THS4509 in figure 17 of the ADS5463-SP data sheet be replaced with the THS4513-SP with no changes to resistor/capacitor values? (assuming same gain is needed)
In out application we will have a DC-coupled input to the THS4513 that is a 50ohm source. My understanding is that the 78.9ohm and 100ohm resistors on the input of the amp in figure 17 of the ADS5463-SP data sheet are there to match the impedance of the source..but 100||78.9 is only 44.1ohm so is my understanding wrong?
Lastly, a very similar circuit drawing is shown in figure 66 of the THS4513-SP data sheet for driving an ADC. However, the resistor values on the input of the amp are slightly different...69.8ohms show in THS4513-SP data sheet vs. 78.9ohm shown in ADS5463 data sheet....which value woudl be correct for using the THS4513-SP to drive the ADS5463-SP ADC?
Any help you can provide for properly driving the ADS5463-SP with the THS4513-SP is greatly appreciated.
The THS4513-SP should do just fine in place of the THS4509. The main difference is that the THS4513 was compensated to be stable at lower gains. This resulted in a bandwidth reduction from 1900MHz to 1600MHz. The core amplifier should be the same though, so the THS4513-SP device can be used in place of the THS4509 as long as you are conscious of the design and stay within the boundries.
For figure 17 of the ADS5463-SP, I believe it should read "AC"-coupled input. In this scenario, the voltage set at the Vocm pin will directly set the output common mode impedance and the input common mode input to be the same. You can use a thevenin equivalent circuit to help solve for the gain at the flatband, Vo/Vin = 20*log(78.6/(78.6+49.9)) * 348/(100+78.6||49.9) = 4.24dB. Of course, if you change your reference point to after the source termination, you would have subtract out the loss to have 4.24dB - (-5.66dB) = 9.9dB or about 10dB.
You are correct about the reason for the 78.9Ω. It is added to terminate the 50Ω source, however it is a little more complicated than 100||78.9. A more detailed derivation of this as well as a spreadsheet to help you calculate the correct value can be found in the following Analog Applications Journal article: http://www.ti.com/analog/docs/litabsmultiplefilelist.tsp?literatureNumber=slyt310&docCategoryId=1&familyId=57
If you want a DC-coupled input, you could use a circuit like figure 63 of the THS4513-SP datasheet. I've attached a TINA-TI schematic as a starting point.
8662.THS4513 DC Coupled.TSC
Let me know if you have any further questions.
Regards,Luke LaPointeHigh Speed Amplifiers
Regards,Luke LaPointeHigh Speed Data Converters
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