AD9251 FDA

Question

FDA input ZIn 
 
 My question. In the datasheet fig 41 shows a recommended differential driver (FDA) interfacing to the ADC diff inputs. It looks like the Zin of the amplifier is appx 50ohms. I have a case where I could drive this FDA with another op amp but using a higher load than 50ohms would be easier. Most op amps cannot drive 50ohms directly so I would need to use a series 50ohm to limit current. Is it possible to reconfigure the FDA input a bit to make it maybe 200-500 ohms? Any circuit suggestions? Freq of operation is <= 1MHz.

Hooman · Accepted Answer

Hi Dennis, 
 If all you're trying to do is to raise the diff amp for higher Zin, you could employ the ADI Diff-Amp Calculator for that: 
 https://www.analog.com/en/design-center/interactive-design-tools/adi-diffampcalc.html 
 I've configured the ADA4938 for Zin = 500ohm below: 
 
 All you do is enter your Rs=500ohm, enter your Vocm (1V for example), supply voltages (+/-5V for example), and enter the Target Gain (1V/V) with some starting value for Rg (e.g. 1kohm). The tool computes the other stuff. 
 Here is my diff-amp calculator file that you can start with if you like: 
 1586.ADA4938 EZ 4_4_22.zip 
 I've shown this in LTspice simulation as well below: 
 
 Here is the LTspice file if you're interested to play with (Extract All before you open in LTspice): 
 6567.ADA4938 EZ Rin_500ohm Setup 4_4_22.zip 
 Regards, 
 Hooman)

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AD9251 FDA