AD9251 FDA

Thanks for your interest in AD9251. Because this is more of an amplifier question, I've moved it to the "ADC Drivers" Q&A.

Thank you.

Doug

Hi rf_engineer_5,

The AD9251 ADC diff input is a switched capacitor arrangement:

According to AN-742 Application Note, the ADC input impedance will have Real and Imaginary components that vary between the Track to Hold mode. From this application note, we have the following plot for AD9236 ADC (cannot find a similar plot for AD9251 that you've asked about, but there should be similarities):

The important parameter is the Track mode impedance. From Figure 11 of the application note, shown below, that'd be 1.57kohm @ 120MHz:

Your application of only 1MHz signals will have a much higher input impedance than what's shown for 120MHz to be 1.57kohm.

The RC (33ohm and 10pF) of Figure 41 (AD9251 datasheet) are recommended components for best settling at the ADC input.

I hope I've answered your questions.

Regards,

Hooman

Hello Hooman,

Thanks for the reply.  This is not exactly what I was going after.  I was interested in reconfiguring the diff amp input so that it's not 50ohm.  I would like to raise it higher so that I could drive it direct with an op amp.   I was wondering how to configure the op amp Zin to be maybe 200-500ohms?  The gain of the diff amp can be one, or very low.  

dennis

Hello Hooman,

Thanks for the reply.  This is not exactly what I was going after.  I was interested in reconfiguring the diff amp input so that it's not 50ohm.  I would like to raise it higher so that I could drive it direct with an op amp.   I was wondering how to configure the op amp Zin to be maybe 200-500ohms?  The gain of the diff amp can be one, or very low.  

dennis

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:

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):

ADA4938 EZ Rin_500ohm Setup 4_4_22.zip

Regards,

Hooman

Hello Hooman,

This is exactly what i wanted to know.  Didn't know there was a program to do all of the heavy lifting.  Glad there is.  

thanks

dennis