how much C can the PGA281 have. I modified the eval board to 100nf and it oscillated.
Randy
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Hello Randy,
The PGA281 data sheet specifies a typical capacitive load drive of 500 pF. If you connect a larger capacitor than that to the output, stabilization with isolation resistors will be required.
I recommend that you watch our new TI Precision Labs video series on op amp stability to better understand this issue. Specifically, Stability 5 provides a method for stabilizing op amps with an isolation resistor in series with the output.
A SPICE model is available for the PGA281 as well, which allows you to easily test for stability and functionality in TINA-TI.
Best regards,
Ian Williams
Linear Applications Engineer
Precision Analog - Op Amps
Hello Randy,
My apologies. The title of your thread clearly states that the 100nF is on the input.
There are component footprints for an input filter on the PGA281EVM, but from the factory there are only 0-ohm resistors in the signal path. That being said, in TIPD135 I used the following values and didn't encounter any oscillation:
I recommend testing the response of your input filter in TINA to verify the differential and common-mode cutoff and ensure that you don't have any gain peaking.
Best regards,
Ian Williams
Hi Ian,
Yes I may try Tina soon. I am using Ltspice and made up an instrumentation amplifier for my simulations. I may need Tina for the correct model to break the loop and check stability.
It is true that the capacitive loads are to 500pf max. I am driving a differential pair that is just resistive so I am thinking the output side is OK.
The input impedance is ~1G and the input capacitance spec is 12pf. I am thinking my problem of oscillation is on the input side. I am using the eval-board(s). One is 0.100 Rsense and the other is 0.010 for Rsense. The evaluation board has both an input filter and output filter designed in.
When I tried to set the input filter to 20kHz, it oscillated with both increased C and increased R. I tried them individually, one at a time thinking the input may not like to have more C hanging on the differential input(s). I may try to isolate the C by adding 50 ohms to both input.
I will be looking at the front end tomorrow and do need help. Is there any TI person that I can speak with by phone? Is there a TI FAE for Honeywell in Redmond WA? I will check My-TI tomorrow. Thank you,
Randy
I am not sure if this attached correctly. It is a file that shows input filters with 75ohms and not 0 ohms.
Perhaps this is one of the problems. Strange that the documentation goes into great lengths to describe the filter cutoff freq. I am a little confused and assume that I need to let R=0 ??
Randy