Dear Sir,

Kindly find the attachment ,

I plan to use the AD797 and discrete scheme in the in front of AD9257 where 1.8K and 1.5K used to attenuate the input signal as well as common mode voltage add for ADC input.

1.8K and 1.5K provides near to -7dB attenuation for input signal.

i am worry about internal resistance of ADC ,whether the internal resistance affect my attenuation resistors values (1.5K|| Rin of ADC).

Is it change the attenuation level ??

My input frequency is 500Khz only, So the AD797 is suitable for that.

Kindly suggest the solution.

With regards,

Selvakumar

Hi Selvakumar,

In your calculating Ztot, it looks like you are directly adding Zreal and Zimag (negative). Zreal and Zimag are orthogonal quantities in the real-imaginary plane. As such, to calculate their combined magnitude you would need to take the square root of the sum of the squares. Ztot = SQRT(Zreal^2 + Zimag^2).

But, at 300kHz to 500kHz you do not need to worry about accounting for the 5.2kOhm differential resistance. The 5.2kOhm differential input resistance is an equivalent resistance that is valid at input frequencies in the 10s of MHz through a few hundred MHz. The actual input circuit is not a simple lumped parallel RC. Please see this link for a little more information: Differential Input Resistance .

At 300kHz to 500kHz, you will not see attenuation due to the input impedance of the ADC. You do not need to account for the ADC Zin at 300kHz to 500kHz.

Please let me know if you need more information.

Thanks,

Doug