Part Number: OPA855
Other Parts Discussed in Thread: OPA859, LMH6881
Tool/software:
Hi Team,
On page 21 of the OPA855 datasheet an optical front end input stage is described.
There are two reference voltages here:
1. which criteria are important for the 3.8V reference?
2. which criteria are important for the 3.25V reference?
3. why was an expensive OPA859 used for the 3.25V reference?
Best regards
Martin
Hi Martin,
The 3.8V reference can be a low cost reference, But please note it's noise and the error will show up at the OPA855 output.
The 3.25V reference also can be a low cost reference but it need to be buffered with high bandwidth opamp like OPA859. The reason for this is that though the voltage is constant, the current is not. So it has to provide high frequency current when the output of the FDA swings at the high frequency.
If you want to eliminate the buffer, ladder of parallel decoupling capacitor can also be used at the output of the reference source to provide low impedance source path for high freq signal. In this case the capacitor bank will provide any high freq current which is needed when the FDA output swings.
Here one thing to note is that the capacitor at higher frequency can act as an inductor also so proper set of parallel capacitors need to be used in order to make it the impedance low for all frequency of interest.
For OCM of LMH6881 should be drive with low noise source and a decap should be used there as well
Regards
Anant
