AD549: Higher noise

Document created by analog-archivist Employee on Feb 23, 2016
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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.