Datasheet

"#$
SBOS266E − JUNE 2003 − REVISED AUGUST 2008
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18
open-loop gain amplifier like the OPA832 can be very
susceptible to decreased stability and closed-loop re-
sponse peaking when a capacitive load is placed direct-
ly on the output pin. When the primary considerations
are frequency response flatness, pulse response fideli-
ty, and/or distortion, the simplest and most effective
solution is to isolate the capacitive load from the feed-
back loop by inserting a series isolation resistor be-
tween the amplifier output and the capacitive load.
The Typical Characteristic curves show the recom-
mended R
S
versus capacitive load and the resulting fre-
quency response at the load. Parasitic capacitive loads
greater than 2pF can begin to degrade the performance
of the OPA832. Long PC board traces, unmatched
cables, and connections to multiple devices can easily
exceed this value. Always consider this effect carefully,
and add the recommended series resistor as close as
possible to the output pin (see the Board Layout Guide-
lines section).
The criterion for setting this R
S
resistor is a 1dB peaked
frequency response at the load. Increasing the noise
gain will also reduce the peaking (see Figure 7).
DISTORTION PERFORMANCE
The OPA832 provides good distortion performance into
a 150Ω load. Relative to alternative solutions, it pro-
vides exceptional performance into lighter loads and/or
operating on a single +3.3V supply. Generally, until the
fundamental signal reaches very high frequency or
power levels, the 2nd-harmonic will dominate the distor-
tion with a negligible 3rd-harmonic component. Focus-
ing then on the 2nd-harmonic, increasing the load im-
pedance improves distortion directly. Remember that
the total load includes the feedback network; in the non-
inverting configuration (see Figure 3) this is sum of R
F
+ R
G
, while in the inverting configuration, only R
F
needs
to be included in parallel with the actual load.
Figure 9 shows the 2nd- and 3rd-harmonic distortion
versus supply voltage. In order to maintain the input sig-
nal within acceptable operating range, the input com-
mon-mode voltage is adjusted for each supply voltage.
For example, the common-mode voltage is +2V for a
single +5V supply, and the distortion is −66.5dBc for the
2nd-harmonic and −74.6dBc for the 3rd-harmonic.
−66
−67
−68
−
69
−70
−
71
−
72
−73
−
74
−75
−
76
Supply Voltage (V)
Harmonic Distortion (dBc)
5.5
5.0
4.5
4.0
3.5
3.0
2.5
2.0
1.5
1.0
0.5
Common−Mode Voltage (V)
567891011
G=+2V/V
R
L
=500
Ω
V
O
=2V
PP
f=5MHz
Common−Mode Voltage
Right Scale
2nd−Harmonic
Left Scale
3rd−Harmonic
Left Scale
Figure 9. 5MHz Harmonic Distortion vs Supply
Voltage
NOISE PERFORMANCE
Unity-gain stable, rail-to-rail (RR) output, voltage-feed-
back op amps usually show a higher input noise volt-
age. The 9.2nV/√Hz input voltage noise for the OPA832
however, is much lower than comparable amplifiers.
The input-referred voltage noise and the two input-re-
ferred current noise terms (2.8pA/√Hz) combine to give
low output noise under a wide variety of operating
conditions. Figure 10 shows the op amp noise analysis
model with all the noise terms included. In this model,
all noise terms are taken to be noise voltage or current
density terms in either nV/√Hz or pA/√Hz.
4kT
R
G
R
G
R
F
R
S
OPA832
I
BI
E
O
I
BN
4kT = 1.6E
−
20J
at 290
_
K
E
RS
E
NI
4kTR
S
√
4kTR
F
√
Figure 10. Noise Analysis Model
The total output spot noise voltage can be computed as
the square root of the sum of all squared output noise
voltage contributors. Equation 1 shows the general
form for the output noise voltage using the terms shown
in Figure 10:
E
O
+
ǒ
E
NI
2
)
ǒ
I
BN
R
S
Ǔ
2
) 4kTR
S
Ǔ
NG
2
)
ǒ
I
BI
R
F
Ǔ
2
) 4kTR
F
NGǸ
(1)