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Part Number: LMH6629
Good afternoon, I am using the subject amplifier in a transimpedance application with a photodiode. Currently, in addition to the transmipedace stage, we have a second gain stage and then a third stage with a VGA. We are trying to get rid of the second and third stages to combine the variable gain function into the transimpedance stage by using a device such as the TSA3357 part or similiar. See attached...
I am hoping to use the switch to bring in various feedback resistors in to modify the gain. I'm concerned about capacitance affecting the BW of the opamp. What else should I be concerned about? Are there other TI switches that would work better?
Jacob Freet High Speed Amplifiers
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In reply to Jacob%20Freet:
Thanks for your prompt reply! I will review the datasheet the OPA857 part. Concerning the TSA3357 switch, the specs for capacitance of the common and NO contacts in reference to ground as shown in Figure 15. Correct me if I am wrong, but it seems like this capacitance may be an issue with signal distortiion, but not necessary BW? I was looking for a spec in the datasheet for a spec for the capacitance across the switch (from the NO contact to Common), which would seem to affect BW more?
In reply to Robert Walker63:
The switch PN is actually TS5A3357, sorry for the confusion. My question is concerning the switch specifications on capacitance. The capacitance specs appear to be given in reference to ground for the terminals of the switch. I am curious about any parasitic capacitance across the NO and COM terminals. There is no spec for that in the datasheet, but if you look at the spice file, it appears to be 1.47pF? Could you confirm this? Also, am I correct in assuming that the capacitance to ground is more of a stability issue, whereas parasitic capacitance across the switch terminals would do more to reduce the BW?
I will keep these suggestions in mind for this design. Thanks for the feedback!
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