Datasheet
NOISE PERFORMANCE
DC ACCURACY AND OFFSET CONTROL
4kT
R
G
R
G
R
F
R
S
1/2
OPA2889
I
BI
E
O
I
BN
4kT=1.6E 20J-
at+290 K°
E
RS
E
NI
4kTR
S
Ö
4kTRFÖ
± ´ ± ´
± ± ´ m
OS(MAX)
(NG V ) (R I )
= (2 5mV) (750 A)
= 10.6mVwith (NG=noninvertingsignalgain)
OS(MAX) F
´ W
± -
0.75
E =
O
[ ]E +(I R ) +4kTR NG +(I R ) +4kTR NG
2 2 2 2
NI BN S S BI F F
E =
N
E +(I R ) +4kTR + +( )
NI BN S
2 2 2
S
4kTR
NG
F
I R
NG
BI F
(6)
OPA2889
www.ti.com
....................................................................................................................................................... SBOS373B – JUNE 2007 – REVISED AUGUST 2008
previously recommend maximum value of 400 Ω .
Keeping both (R
F
|| R
G
) and the noninverting input
High slew rate, unity-gain stable, voltage-feedback op
source impedance less than 400 Ω satisfies both
amps usually achieve the slew rate at the expense of
noise and frequency response flatness
a higher input noise voltage. However, the 8.4nV/ √ Hz
considerations. Because the resistor-induced noise is
input voltage noise for the OPA2889 is much lower
relatively negligible, additional capacitive decoupling
than that of comparable amplifiers. The input-referred
across the bias current cancellation resistor (R
B
) for
voltage noise, and the two input-referred current
the inverting op amp configuration of Figure 60 is not
noise terms, combine to give low output noise under
required.
a wide variety of operating conditions. Figure 61
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
The balanced input stage of a wideband
in either nV/ √ Hz or pA/ √ Hz.
voltage-feedback op amp allows good output dc
accuracy in a wide variety of applications. The
power-supply current trim for the OPA2889 gives
even tighter control than comparable amplifiers.
Although the high-speed input stage does require
relatively low ± 0.75 µ A input bias current, the close
matching between them may be used to reduce the
output dc error caused by this current. The total
output offset voltage may be reduced by matching the
dc source resistances appearing at the two inputs.
This matching reduces the output dc error resulting
from the input bias currents to the offset current times
the feedback resistor. Evaluating the configuration of
Figure 50 , and using worst-case +25 ° C input offset
voltage and current specifications, gives a worst-case
output offset voltage equal to:
Figure 61. Op Amp Noise Analysis Model
The total output spot noise voltage can be computed
as the square root of the sum of all squared output
A fine-scale output offset null, or dc operating point
noise voltage contributors. Equation 5 shows the
adjustment, is often required. Numerous techniques
general form for the output noise voltage using the
are available for introducing dc offset control into an
terms shown in Figure 61 .
op amp circuit. Most of these techniques eventually
reduce to adding a dc current through the feedback
resistor. In selecting an offset trim method, one key
(5)
consideration is the impact on the desired signal path
frequency response. If the signal path is intended to
Dividing this expression by the noise gain (NG = (1 +
be noninverting, the offset control is best applied as
R
F
/R
G
)) gives the equivalent input-referred spot noise
an inverting summing signal to avoid interaction with
voltage at the noninverting input, as shown in
the signal source. If the signal path is intended to be
Equation 6 .
inverting, applying the offset control to the
noninverting input may be considered. However, the
dc offset voltage on the summing junction sets up a
dc current back into the source that must be
considered. Applying an offset adjustment to the
Evaluating these two equations for the OPA2889
inverting op amp input can change the noise gain and
circuit and component values (see Figure 50 ) gives a
frequency response flatness. For a dc-coupled
total output spot noise voltage of 18.2nV/ √ Hz and a
inverting amplifier, Figure 62 shows one example of
total equivalent input spot noise voltage of 9.1nV/ √ Hz.
an offset adjustment technique that has minimal
This total includes the noise added by the bias
impact on the signal frequency response. In this
current cancellation resistor (350 Ω ) on the
case, the dc offsetting current is brought into the
noninverting input. This total input-referred spot noise
voltage is slightly higher than the 8nV/ √ Hz
specification for the op amp voltage noise alone. This
result is the case as long as the impedances
appearing at each op amp input are limited to the
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