OPA2673 Datasheet, PDF (29/40 Pages) Texas Instruments – Dual, Wideband, High Output Current Operational Amplifier with Active Off-Line Control

OPA2673
www.ti.com..................................................................................................................................................... SBOS382A – JUNE 2008 – REVISED OCTOBER 2008
E_N =
E_NI² + (I_BNR_S)² + 4kTR_S +
I_BIR_F
NG
2
+
4kTR_F
NG
(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
op amp noise model for the differential configuration.
I_N
Driver
E_N
R_S
E_RS
Ö4kTR_S
I_I
R_F
Ö4kTR_F
R_G
E_O2
Ö4kTR_G
R_F
Ö4kTR_F
I_N
R_S
E_N
I_I
E_RS
Ö4kTR_S
As a reminder, the differential gain is expressed as:
G_D = 1 +
2 ´ R_F
R_G
(18)
The output noise voltage can be expressed as shown
below:
E_O² = 2 ´ G_D2 ´ E_N² + (I_N ´ R_S)² + 4kTR_S + 2(I_IR_F)² + 2(4kTR_FG_D)
(19)
Dividing this expression by the differential noise gain,
G_D = (1 + 2R_F/R_G), gives the equivalent input-referred
spot noise voltage at the noninverting input, as shown
in Equation 20.
E_N =
2´
E_N² + (I_N ´ R_S)² + 4kTR_S
+2
I_I
R_F
G_D
2
+2
4kTR_F
G_D
(20)
Evaluating this equation for the OPA2673 circuit and
component values shown on the front page gives a
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.
DC Accuracy and Offset Control
A current-feedback op amp such as the OPA2673
provides exceptional bandwidth in high gains, giving
fast pulse settling but only moderate dc accuracy.
The Electrical Characteristics show an input offset
voltage comparable to high-speed, voltage-feedback
amplifiers; however, the two input bias currents are
somewhat higher and are unmatched. While bias
current cancellation techniques are very effective with
most voltage-feedback op amps, they do not
generally reduce the output dc offset for wideband
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
worst-case +25°C input offset voltage and the two
input bias currents, gives a worst-case output offset
range equal to:
V_OS = ±(NG ´ V_IO(MAX)) ± (I_BN ´ R_S/2 ´ NG) ± (I_BI ´ R_F)
where NG = noninverting signal gain
= ±(4 ´ 7mV) + (25mA ´ 25W ´ 4) ± (402W ´ 48mA)
= ±28mV ± 2.5mV ± 19.3mV
V_OS = ±49.8mV (max at +25°C)
(21)
Figure 85. Differential Op Amp Noise Analysis
Model
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