Datasheet

DISTORTION PERFORMANCE
E
O
E
NI
2
I
BN
R
S
2
4kTR
S
NG
2
I
BI
R
F
2
4kTR
F
NG
(1)
E
N
E
NI
2
I
BN
R
S
2
4kTR
S
I
BI
R
F
NG
2
4kTR
F
NG
(2)
NOISE PERFORMANCE
DC ACCURACY AND OFFSET CONTROL
4kT
R
G
R
G
R
F
R
S
1/3
OPA3832
I
BI
E
O
I
BN
4kT=1.6E 20J-
at290 K°
E
RS
E
NI
4kTR
S
Ö
4kTR
F
Ö
OPA3832
SBOS370A – DECEMBER 2006 – REVISED AUGUST 2008 ............................................................................................................................................ www.ti.com
The total output spot noise voltage can be computed
as the square root of the sum of all squared output
The OPA3832 provides good distortion performance
noise voltage contributors. Equation 1 shows the
into a 150 Ω load. Relative to alternative solutions, it
general form for the output noise voltage using the
provides exceptional performance into lighter loads
terms shown in Figure 55 :
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 distortion with a negligible 3rd-harmonic
Dividing this expression by the noise gain
component. Focusing then on the 2nd-harmonic,
(NG = (1 + R
F
/R
G
)) gives the equivalent input-referred
increasing the load impedance improves distortion
spot noise voltage at the noninverting input, as shown
directly. Remember that the total load includes the
in Figure 55 :
feedback network; in the noninverting configuration
(see Figure 47 ) this is the sum of R
F
+ R
G
, while in
the inverting configuration, only R
F
needs to be
included in parallel with the actual load.
Evaluating these two equations for the circuit and
component values shown in Figure 46 gives a total
High slew rate, unity-gain stable, voltage-feedback op
output spot noise voltage of 18.8nV/ √ Hz and a total
amps usually achieve their slew rate at the expense
equivalent input spot noise voltage of 9.42nV/ √ Hz.
of a higher input noise voltage. The 9.2nV/ √ Hz input
This total includes the noise added by the resistors.
voltage noise for the OPA3832, however, is much
This total input-referred spot noise voltage is not
lower than comparable amplifiers. The input-referred
much higher than the 9.2nV/ √ Hz specification for the
voltage noise and the two input-referred current noise
op amp voltage noise alone.
terms (2.2pA/ √ Hz) combine to give low output noise
under a wide variety of operating conditions.
Figure 55 shows the op amp noise analysis model
The balanced input stage of a wideband
with all the noise terms included. In this model, all
voltage-feedback op amp allows good output dc
noise terms are taken to be noise voltage or current
accuracy in a wide variety of applications. The
density terms in either nV/ √ Hz or pA/ √ Hz.
power-supply current trim for the OPA3832 gives
even tighter control than comparable products.
Although the high-speed input stage does require
relatively high input bias current (typically 5 µ A out of
each input terminal), the close matching between
them may be used to reduce the output dc error
caused by this current. This configuration matches
the dc source resistances appearing at the two
inputs. Evaluating the configuration of Figure 48
(which has matched dc input resistances), using
worst-case +25 ° C input offset voltage and current
specifications, gives a worst-case output offset
voltage equal to:
• (NG = noninverting signal gain at dc)
• ± (NG × V
OS(MAX)
) + R
F
× I
OS(MAX)
)
• = ± (2 × 80mV) + (400 Ω × 1.5 µ A)
Figure 55. Noise Analysis Model
• = – 15.4mV to +16.6mV
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Product Folder Link(s): OPA3832