AD8129: Higher gain

Document created by analog-archivist Employee on Feb 23, 2016
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The customer wants to use the AD8129. They need a  gain=250. in the datasheet
always there talk of maxgain=10 and the resistors for feedback are given only
for maxgain of 10 (page 12/Datasheet). Are there any limitations for higher
gains higher than 10???


The data sheet refers to a minimum gain of 10 in order to achieve stability --
a closed-loop gain of 10 results in about 56 degrees of phase margin.
As you go to higher gains, the amplifier gets more stable.  The AD8129 is the
best choice of differential receivers for high gain applications since it has a
high gain-bandwidth product (about 2 GHz).  The AD8129 behaves the same way as
a conventional voltage feedback op-amp with regard to gain-bandwidth product,
so as you increase the closed-loop gain, you reduce the bandwidth
proportionally.  To estimate the bandwidth of a single stage at a gain of 250,
take the bandwidth at a gain of 100 and multiply it by 0.4.  If you need more
bandwidth you can reduce the gain of the AD8129 (not below 10) and add a
single-ended op-amp stage following it to make up the difference.

From a practical standpoint, you should not use a large feedback resistor to
achieve a large gain.  You can make Rg quite small instead.  For a gain of 251,
I recommend you use Rf = 2.5K and Rg = 10.  You should also be sure to void all
copper from the planes under the summing node (under all of the traces that
connect to the feedback pin) in order to minimize the stray capacitance at that