Other Parts Discussed in Thread: OPA4350
Hello,
I have inherited this radiation detection board design at work. None of the original designers are with us and very little documentation exists. The design has been through several revisions and still 30% of the boards do not work as desired, or they work at our facility when we test them but do not work at customer's site.
A PIN diode (PDB-C156-ND) is used as the sensor, followed by an LMP7721 as the first stage transimpedance amplifier. I first suspected layout and noise issues, because there were no guard rings on the board. The whole circuit is enclosed in an aluminum shield, but no guard rings. As I investigated more, now I suspect the feedback capacitor and resistors values as well.
This stage is followed by a band-pass filter. Of course I found cut-off frequency settings to be a little off and as I changed the values performance improved significantly.
Though I still cannot figure out how previous designers of this board calculated capacitor and resistor values for LMP7721, and I need a second opinion with that since my own calculations yield different values.
You can see the LMP7721 part of the circuit below, where;
C19 = 0.6pF, R3 = 10Mohm, R17 = 100Kohm & R18 = 49.9Kohm
Using equation 8 from the datasheet (page 19) and assuming that;
Cs = Cin + Cj + Cstray, where Cin is LMP7721 input capacitance (11pF), Cj is junction capacitance of photodiode (10pF) and assuming 3pF for any stray capacitance. I have Cf ~= 0.4pF. Where if I Do not change R17 & R18, then C19 should be 0.8pF, or if I change R17 & R18 to 50K and 10K, C19 nan be 2pF.
So I have two questions;
1- Do you think the current values will work fine and I should just leave them the way they are, or shall I replace them with any of these configurations I mentioned above?
2- Do you suggest that I change the layout and add a guard ring for LMP7721?
Thanks for your help,
May Varza