OPA2674 Datasheet, PDF (24/33 Pages) Burr-Brown (TI) – Dual Wideband, High Output Current Operational Amplifier with Current Limit

OPA2674
SBOS270A − AUGUST 2003 − REVISED MAY 2006
R_S
E_RS
√4 kTR_S
I_N
Driver
E_N
I_I
R_F
√4kTR_F
R_G
E_O2
√4kTR_G
R_F
√ 4kTR_F
I_N
R_S
E_N
I_I
E_RS
√4 kTR_S
Figure 14. Differential Op Amp Noise Analysis
Model
As a reminder, the differential gain is expressed as:
G_D
+
1)²
R_F
R_G
(19)
The output noise voltage can be expressed as shown below:
(20)
Ǹ ǒ Ǔ eO2 +
2 GD2
ǒ Ǔ eN2 ) iN
2
RS ) 4kTRS
) 2ǒiIRFǓ² ) 2ǒ4kTRFGDǓ
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 21.
(21)
Ǹ ǒ Ǔ ǒ Ǔ ǒ Ǔ e_N +
2
e_N2 ) ^ǒi_N
R
Ǔ²
S
)
4kTR_S
)2
i_I
R_F
G_D
2
)2
4kTR_F
G_D
Evaluating this equation for the OPA2674 circuit and
component values of Figure 5 gives a total output spot
noise voltage of 31.0nV/√Hz and a total equivalent input
spot noise voltage of 3.5nV/√Hz.
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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 OPA2674
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 1, using 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 × 4.5mV) ± (30µA × 25Ω × 4) ± (402Ω × 35µA)
= ±18mV ± 3mV ± 14mV
V_OS = ±35.0mV (max at 25°C)
POWER CONTROL OPERATION (SO-14 ONLY)
The OPA2674I-14D provides a power control feature that
may be used to reduce system power. The four modes of
operation for this power control feature are full-power,
power cutback, idle state, and power shutdown. These
four operating modes are set through two logic lines A0
and A1. Table 3 shows the different modes of operation.
Table 3. Power Control Mode of Operation
MODE OF
OPERATION
A1
A0
Full-Power
1
1
Power Cutback
1
0
Idle State
0
1
Shutdown
0
0
The full-power mode is used for normal operating
condition. The power cutback mode brings the quiescent
power to 13.5mA. The idle state mode keeps a low output
impedance but reduces output power and bandwidth. The
shutdown mode has a high output impedance as well as
the lowest quiescent power (1.0mA).
If the A0 and A1 pins are left unconnected, the
OPA2674I-14D operates normally (full-power).