Datasheet

E =
I
E +(I xR ) +4kTR
NI BN S S
+(I xR ) +4kTR
BI F F
2 2
2
NG
NG
2
R
G
R
F
R
S
E
2
O
Driver
E
RS
E
N
I
N
I
I
Ö4kTR
S
Ö4kTR
F
R
F
R
S
E
RS
E
N
I
I
I
N
Ö4kTR
S
Ö4kTR
G
Ö4kTR
F
G =1+
D
2 R´
F
R
G
E =
O
2
E +(I R ) +4kTR
N N S S
´
2 2
2 G´
D
´
2
+2(I R ) +2(4kTR G )
I F F
D
2
E =
I
E +(I R ) +4kTR
N N S S
´
2 2
4kTR
F
G
D
R
F
G
D
2
+22 ´ +2
I
I
V = (NG V ) (I R /2 NG) (I R )
whereNG=noninvertingsignalgain
± ´ ± ´ ´ ± ´
IO(MAX) BN S BI F
OS
= (4 7mV)+(25 A 25 4) (402 48 A)
= 28mV 2.5mV 19.3mV
V = 49.8mV(maxat+25 C)
± ´ m ´ W ´ ± W ´ m
± ± ±
± °
OS
OPA2673
www.ti.com SBOS382F –JUNE 2008–REVISED MAY 2010
Dividing this expression by the noise gain [NG = (1 +
R
F
/R
G
)] gives the equivalent input-referred spot noise
voltage at the noninverting input, as shown in
Equation 17.
(17)
Evaluating these two equations for the OPA2673
circuit and component values of Figure 76 gives a
total output spot noise voltage of 18nV/√Hz and a
total equivalent input spot noise voltage of 4.5nV/√Hz.
This total input-referred spot noise voltage is higher
than the 2.4nV/√Hz specification for the op amp
voltage noise alone. This result reflects the noise
added to the output by the inverting current noise
times the feedback resistor. If the feedback resistor is
reduced in high-gain configurations (as suggested
previously), the total input-referred voltage noise
given by Equation 17 approaches only the 2.4nV/√Hz
of the op amp. For example, going to a gain of +8V/V
using R
F
= 250Ω gives a total input-referred noise of
2.8nV/√Hz.
Differential Noise Performance
Because the OPA2673 is used as a differential driver
in PLC applications, it is important to analyze the
noise in such a configuration. See Figure 85 for the
Figure 85. Differential Op Amp Noise Analysis
op amp noise model for the differential configuration.
Model
As a reminder, the differential gain is expressed as:
DC Accuracy and Offset Control
(18)
A current-feedback op amp such as the OPA2673
provides exceptional bandwidth in high gains, giving
The output noise voltage can be expressed as shown
fast pulse settling but only moderate dc accuracy.
below:
The Electrical Characteristics show an input offset
voltage comparable to high-speed, voltage-feedback
amplifiers; however, the two input bias currents are
(19)
somewhat higher and are unmatched. While bias
Dividing this expression by the differential noise gain, current cancellation techniques are very effective with
G
D
= (1 + 2R
F
/R
G
), gives the equivalent input-referred most voltage-feedback op amps, they do not
spot noise voltage at the noninverting input, as shown generally reduce the output dc offset for wideband
in Equation 20. current-feedback op amps. Because the two input
bias currents are unrelated in both magnitude and
polarity, matching the input source impedance to
reduce error contribution to the output is ineffective.
Evaluating the configuration of Figure 76, using a
(20)
worst-case +25°C input offset voltage and the two
Evaluating this equation for the OPA2673 circuit and
input bias currents, gives a worst-case output offset
component values shown on the front page gives a
range equal to:
total output spot noise voltage of 72.3nV/√Hz and a
total equivalent input spot noise voltage of
18.4nV/√Hz.
In order to minimize the noise contributed by I
N
, it is
recommended to keep the noninverting source
impedance as low as possible.
(21)
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