my bro designed a tia circuit using opa847,the pin has a cap of 25 pf ,two pin consists a parall cap of 50 pf,now i need to make the circuit have 200Mhz bandwidth,and a flat freq response,what pacakage and value of the Rf and Cf should i choose? and is the layout reasonable?
thanks anyway!
A couple of schematic things,
1. The 220 on the V+ node is for bias current cancellation to improve output DC error, you are AC coupled, reduce that R to 20ohm and get rid of the cap in parallel
2. with 50pF source, you can get a 220MHz Butterworth with 220ohm Rf setting the Cf to 4.6pF.
I went ahead and ran some sims with the OPA847, not looking very good - pretty peaky,
You might want to consider the much faster OPA855, here I tuned that up for 499ohm gain,
+/-2.5V supplies, here it the TINA V9 file,
using this eqa from TI reference, the Cf should be 4.308pF,it's a little different from yours,i guess your is better,could you tell me how you get the calue of Cf?
hamamatsu, g12182_003K is the name of photodiodes. The reason i want to replace opa847 by opa657 is that i think the 657's phase margin is larger than opa847,which will make it easier to choose rf and cf to make the circiut stable,any wrong about my thoughts? thanks anyway!
my new design with opa847!
Here is a theoretical solution, you can see the peaking, going up to 4.6pF helped -
This is now flat through 120MHz and you can postfilter the peaking if you want,
That was just an oversight in changing the sim file, normally the macro models don't pick up much AC response changes with supply voltage, but the OPA847 is a transistor level model and going to +/-5V it looks a lot better, actually pushed up to 499ohm gain with good flatness through 120MHz,
And the V9 file, OPA847 low gain high Cs Zt.TSC
