Datasheet

DISTORTION PERFORMANCE
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)
E
N
+ E
NI
2
)
ǒ
I
BN
R
S
Ǔ
2
) 4kTR
S
)
ǒ
I
BI
R
F
NG
Ǔ
2
)
4kTR
F
NG
Ǹ
(2)
NOISE PERFORMANCE
DC ACCURACY AND OFFSET CONTROL
4kT
R
G
R
G
R
F
R
S
1/2
OPA2832
I
BI
E
O
I
BN
4kT = 1.6E
−
20J
at 290
_
K
E
RS
E
NI
4kTR
S
√
4kTR
F
√
OPA2832
www.ti.com ............................................................................................................................................. SBOS327C – FEBRUARY 2005 – REVISED AUGUST 2008
The OPA2832 provides good distortion performance The total output spot noise voltage can be computed
into a 150 Ω load. Relative to alternative solutions, it as the square root of the sum of all squared output
provides exceptional performance into lighter loads noise voltage contributors. Equation 1 shows the
and/or operating on a single +3.3V supply. Generally, general form for the output noise voltage using the
until the fundamental signal reaches very high terms shown in Figure 71 :
frequency or power levels, the 2nd-harmonic will
dominate the distortion with a negligible 3rd-harmonic
component. Focusing then on the 2nd-harmonic,
increasing the load impedance improves distortion
Dividing this expression by the noise gain
directly. Remember that the total load includes the
(NG = (1 + R
F
/R
G
)) will give the equivalent
feedback network; in the noninverting configuration
input-referred spot noise voltage at the noninverting
(see Figure 62 ) this is sum of R
F
+ R
G
, while in the
input, as shown in Figure 71 :
inverting configuration, only R
F
needs to be included
in parallel with the actual load. Running differential
suppresses the 2nd-harmonic, as shown in the
differential typical characteristic curves.
Evaluating these two equations for the circuit and
component values shown in Figure 61 will give a total
output spot noise voltage of 19.3nV/ √ Hz and a total
High slew rate, unity-gain stable, voltage-feedback op
equivalent input spot noise voltage of 9.65nV/ √ Hz.
amps usually achieve their slew rate at the expense
This is including the noise added by the resistors.
of a higher input noise voltage. The 9.2nV/ √ Hz input
This total input-referred spot noise voltage is not
voltage noise for the OPA2832, however, is much
much higher than the 9.2nV/ √ Hz specification for the
lower than comparable amplifiers. The input-referred
op amp voltage noise alone.
voltage noise and the two input-referred current noise
terms (2.8pA/ √ Hz) combine to give low output noise
under a wide variety of operating conditions.
Figure 71 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 OPA2832 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 is done by matching the
DC source resistances appearing at the two inputs.
Evaluating the configuration of Figure 63 (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 × 7.5mV) + (400 Ω × 1.5 µ A)
• = – 14.4mV to +15.6mV
Figure 71. Noise Analysis Model
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Product Folder Link(s): OPA2832