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Lower resistor values are ideal for low-noise performance
at the cost of increased distortion due to increased load-
ing of the feedback network on the output stage. Higher
resistor values will yield better distortion performance
due to less loading on the output stage but at the cost of
increase in higher output noise.
Improving Stability Using
Feedback Capacitors
When the MAX44206 is configured such that a combina-
tion of parasitic capacitances at the inverting input form
a pole whose frequency lies within the closed-loop band-
width of the amplifier, a feedback capacitor across the
feedback resistor is needed to form a zero at a frequency
close to the frequency of the parasitic pole to recover the
lost phase margin.
Adding larger value feedback capacitors will reduce the
peaking of the amplifier but decreases the closed-loop
-3dB bandwidth.
Layout and Bypass Capacitors
For single-supply applications, it is recommended to place
a 0.1µF NPO or C0G ceramic capacitor within 1/8th of
an inch from the VS+ pin to ground and to also connect
a 10µF ceramic capacitor within 1 inch of the VS+ pin to
GND.
In dual-supply applications, it is recommended to install
0.1µF NPO or C0G ceramic capacitor within 1/8th of an
inch from the VS+ and VS- pins to GND and place 10µF
ceramic capacitors within 1 inch of the VS+ and VS- pins
to GND. Low ESR\ESL NPO capacitors are recommend-
ed for 0.1µF or smaller decoupling capacitors. A 0.1µF or
0.22µF capacitor is a good choice close to VOCM input
pin to ground.
Signal routing into and out of the part should be direct
and as short as possible into and out of the op amp
inputs and outputs. The feedback path should be carefully
routed with the shortest path possible without any para-
sitic capacitance forming between feedback trace and
board power planes. Ground and power planes should be
removed from directly under the amplifier input and output
pins. Also, care should be taken such that there will be no
parasitic capacitance formed around the summing nodes
at the inputs that could affect the phase margin of the part.
Any load capacitance beyond a few picofarads needs to
be isolated using series output resistors placed as close
as possible to the output pins to avoid excessive peaking
or instability.
Driving a Fully Differential ADC
The MAX44206 was designed to drive fully differential
SAR ADCs such as the MAX11905. The MAX11905 is
part of a family of 20-/18-/16-bit, 1.6Msps/1Msps ADCs
that offer excellent AC and DC performance. Figure 8
details a fully differential input to the MAX44206, which
then drives the fully differential MAX11905 ADC inputs
through the ADC input filter shown in the dashed box.
The MAX6126 provides a 3V reference output voltage,
which is fed to the ADC’s reference. The MAX44206’s
common mode (VOCM) is created by dividing down the
reference voltage by a factor of two. A pair of 1kΩ 0.1%
resistors are used for this purpose. The VOCM input is
bypassed to GND with a combination of 2.2µF (X7R) and
0.1µF (NPO) capacitors.
The MAX44206 is connected in a unity-gain configuration.
The input resistors and feedback resistors are all 1kΩ
0.1% resistors. The feedback resistors are bypassed by
a pair of 4.7nF (C0G, 100V) capacitors. These feedback
components roll the amplifier off to about 60MHz corner
frequency.
The ADC input filter uses a pair of 10Ω 0.1% resistors
and a 2.2nF (C0G) capacitor. This input filter assists the
MAX44206’s settling response with the MAX11905’s fast
acquisition window.
Figure 8 was used to test the AC performance in Figures
9 and 10. Data were taken with the input frequencies
at 10kHz on the MAX11905 Evaluation Kit. Figures 9
to 13 detail the results of the MAX11905 Evaluation Kit
(MAX11905DIFEVKIT#) GUI.
MAX44206 180MHz, Low-Noise, Low-Distortion, Fully
Differential Op Amp/ADC Driver
www.maximintegrated.com Maxim Integrated
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