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OPA3S2859-EP: Using the COM_A input vs INA

Part Number: OPA3S2859-EP

Page 1 of the datasheet shows the OPA3S2850 used as transimpedance amplifier with a photo diode connected at the COM_A input and hence allowing the selection of LO, MID and HI gain by programming the internal switches. So far the datasheet is clear. 

On page 4 of the datasheet is the table 5-2. "Select Pin Decoder" and this table shows the possible SEL0/SEL1 combinations for LO/MID and HI gain plus a fourth option where both SEL0/SEL1 are set high, resulting in an external gain setting with all internal switches open. When all switches are set to OPEN, the COM_A input is no longer connected to the input of the amplifier. 

Question 1: For our use-case, we have a photodiode on COM_A (and COM_B) input(s) and program SEL0/SEL1 to have three gain settings in which we do not use the INA- input. Should this INA (and INB) input be left floating in this case? 

Question 2: The general layout example on page 23 shows a use-case for both COM_A and INA, however when you use a photodiode on COM_A, you do not want INA to pick up any noise or have noise coupled from other PCB areas. I'm assuming that this PCB layout on page 23 is only shown to demonstrate all possible use-cases and hence not optimized for the case where specifically COM_A is used as an input.

Thanks for the feedback!



  • Hello Peter,

    1) Correct, you may leave INA- and INB- floating. INA+ and INB+ can be left floating as well if you do not need to add an external bias depending on your supply voltage configuration. 

    2) Yes, that is correct. This layout example was taken from our OPA3S2859RTWEVM which is an evaluation module for testing the device in all the different input/output possibilities including the external gain resistor between IN- and OUT pins as you have mentioned prior to your questions. 

    Thank you,