Search FAQs on the left to see if your question has been answered. Click on the dropdown to view all of the documents associated with the product. If you can't find your question, click on Ask a Question

AD549: Higher noise

I have a problem using the AD549. I see way more noise than what I've expected
from the datasheet.
The configuration is classic negative feedback.
The input is on -Ve pin (2) and the +Ve pin (3) is at 0V.
V- supply is -15V and V+ supply is +15V
The feedback resistor is up to 1G0.
There is no input resistor. The input current goes directly through the
feedback resistor.
10pF across the -Ve (2) and Out (6) pins.
Nulling pot is used (50K0 between pins 1 and 5.

That's it. The application is mostly a photodiode amplifier.


1GOhms resistor is far too big for the feedback resistor. The applications
engineer would recommend not taking the chance to use such a big resistor for
this type of application. There are a lot of implications using such a large
value resistor: 1. The resistor itself will contribute to a noise figure of
about 6uV/sqrHz; 2. Any small parasitic on the PC board could create stability
problems for the amplifier.
Usually we recommend to use a feedback resistor not larger than 10M Ohms and
implement a second stage amplifier for the voltage amplification.