Rainbow Electronics MAX4249_MAX4257 User Manual
Page 10
MAX4249–MAX4257
UCSP, Single-Supply, Low-Noise,
Low-Distortion, Rail-to-Rail Op Amps
10
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Low Distortion
Many factors can affect the noise and distortion that the
device contributes to the input signal. The following
guidelines offer valuable information on the impact of
design choices on Total Harmonic Distortion (THD).
Choosing proper feedback and gain resistor values for
a particular application can be a very important factor
in reducing THD. In general, the smaller the closed-
loop gain, the smaller the THD generated, especially
when driving heavy resistive loads. Large-value feed-
back resistors can significantly improve distortion. The
THD of the part normally increases at approximately
20dB per decade, as a function of frequency.
Operating the device near or above the full-power
bandwidth significantly degrades distortion.
Referencing the load to either supply also improves the
part’s distortion performance, because only one of the
MOSFETs of the push-pull output stage drives the out-
put. Referencing the load to midsupply increases the
part’s distortion for a given load and feedback setting.
(See the Total Harmonic Distortion vs. Frequency graph
in the Typical Operating Characteristics.)
For gains
≥ 10V/V, the decompensated devices
MAX4249/MAX4255/MAX4256/MAX4257 deliver the
best distortion performance, since they have a higher
slew rate and provide a higher amount of loop gain for
a given closed-loop gain setting. Capacitive loads
below 400pF, do not significantly affect distortion
results. Distortion performance remains relatively con-
stant over supply voltages.
Low Noise
The amplifier’s input-referred, noise-voltage density is
dominated by flicker noise at lower frequencies, and by
thermal noise at higher frequencies. Because the ther-
mal noise contribution is affected by the parallel combi-
nation of the feedback resistive network (R
F
|| R
G
,
Figure 1), these resistors should be reduced in cases
where the system bandwidth is large and thermal noise
is dominant. This noise contribution factor decreases,
however, with increasing gain settings.
For example, the input noise-voltage density of the cir-
cuit with R
F
= 100k
Ω, R
G
= 11k
Ω (A
V
= 10V/V) is en =
15nV/
√Hz, en can be reduced to 9nV/√Hz by choosing
R
F
= 10k
Ω, R
G
= 1.1k
Ω (A
V
= 10V/V), at the expense
of greater current consumption and potentially higher
distortion. For a gain of 100V/V with R
F
= 100k
Ω, R
G
=
1.1k
Ω, the en is low (9nV/√Hz).
C
Z
R
F
V
OUT
V
IN
R
G
0
100mV
A
V
= 2V/V
R
F
= R
G
= 10k
Ω
V
IN
=
50mV/div
V
OUT
=
100mV/div
2
µs/div
0
100mV
A
V
= 2
R
F
= R
G
= 100k
Ω
C
Z
= 11pF
50mV/div
100mV/div
V
IN
V
OUT
2
µs/div
Figure 1. Adding Feed-Forward Compensation
Figure 2a. Pulse Response with No Feed-Forward
Compensation
Figure 2b. Pulse Response with 10pF Feed-Forward
Compensation