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Precision Data Converters
Precision Data Converters Forum
ADS8363 driver source impedance
I'm having trouble meeting the 12-ohm driver impedance requirement for the maximum sampling frequency of the ADS8363. The driver is an OPA365 as recommended in the datasheet configured for G=-1 with 2.2k-ohm resistors and 3.3pF feedback compensation. Actual circuit measurements of output impedance cross 12-ohms at about 10MHz input. Does the source impedance requirement not need to be met all the way up to 19MHz (Bandwidth requirement in ADS8363 datasheet)?
I have also simulated the circuit in TINA with improved output impedance characteristics but it still fails to meet 12-ohms @ 19MHz.
Thanks for the help,
You don't necessarily have to match the impedance recommendation. Full speed operation of the ADS8363 (by my calculations) needs to have a GBW of about 75MHz, and the OPA365 is only ~50MHz, so this might not be the best driver amplifier if you need to reach full speed of the device. We'll look over some of our collective notes and get back to you shortly with a couple of options - are your key care abouts more for DC performance or AC?
I'm somewhat new to all of this so it's hard for me to say. Essentially I'm feeding in a square-wave of varying offset and amplitude and need to accurately measure the difference between high level and low level. The frequency is fixed at 1 kHz but the edges also have some valuable information as well.
Could you also share the calculations you're doing that came out with a 75MHz Op-amp as well?
Have a look over the equations in the attached file. This is just a general 'rule of thumb' set of calculations we run to try and find a starting point. 'N' is the number of bits, 16 for the ADS8363. The tAQ is the acquisition time, which is 100ns.
I don't really understand where the last equation comes from (GBW = 4*fZX), however I should be okay to increase the acquisition time to 200 ns which seems to meet the settling time requirement over the bandwidth for the OPA365.
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