OPA2613 Datasheet, PDF (22/27 Pages) Texas Instruments – Dual, Wideband, High Output Current, Operational Amplifier with Current Limit

OPA2613
SBOS249D − JUNE 2003− REVISED APRIL 2004
Figure 12 shows a comparison of 2 • Input voltage to the
capacitor voltage. The transition time is set by the 70V/µs
slew rate for the OPA2613. For this controlled dV/dT, the
charging current into the 5000pF load will be given by:
Slew Rate = I_P/C
Solving for I_P gives:
I_P+ 5000pF 70Vńms + 350mA peak current ₍₁₆₎
6
5
Capacitor Voltage
4
3
2X Input Voltage
2
1
0
−1
70V/µs Slew Rate
−2
−3
−4
−5
−6
Time (100ns/div)
Figure 12. Large-Signal Capacitive Load Drive
At these larger capacitive loads, very low series R will
maintain stabilitybut some R is always required.
DISTORTION PERFORMANCE
The OPA2613 provides good distortion performance into
a 100Ω load on ±6V supplies. Generally, until the
fundamental signal reaches high frequency or power
levels, the 2nd-harmonic dominates the distortion with a
negligible 3rd-harmonic component. Focusing then on the
2nd-harmonic, increasing the load impedance improves
distortion directly. Remember that the total load includes
the feedback networkin the noninverting configuration
(see Figure 1), this is the sum of R_F + R_G, whereas in the
inverting configuration, it is just R_F. Also, providing an
additional supply decoupling capacitor (0.01µF) between
the supply pins (for bipolar operation) improves the
2nd-order distortion slightly (3dB to 6dB).
In most op amps, increasing the output voltage swing
increases harmonic distortion directly. The Typical
Characteristics show the 2nd-harmonic increasing at a
little less than the expected 2x rate whereas the
3rd-harmonic increases at a little less than the expected 3x
rate. Where the test power doubles, the difference
between it and the 2nd-harmonic decreases less than the
expected 6dB, whereas the difference between it and the
3rd-harmonic decreases by less than the expected 12dB.
Operating differentially will suppress the 2nd-order
harmonics below the 3rd.
Operating as a differential I/O stage will also suppress the
2nd-harmonic distortion.
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NOISE PERFORMANCE
Wideband voltage-feedback op amps generally have a
lower output noise than comparable current-feedback op
amps. The OPA2613 offers an excellent balance between
voltage and current noise terms to achieve low output
noise. The input voltage noise (1.8nV/√Hz) is lower than
most unity-gain stable, wideband voltage-feedback op
amps. The op amp input voltage noise and the two input
current noise terms combine to give low output noise
under a wide variety of operating conditions. Figure 13
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 in either nV/√Hz
or pA/√Hz.
E_NI
1/2
R_S
I_BN
OPA2613
E_O
E_RS
√4 kTR_S
4kT
R_G
R_F
R_G
I_BI
√4kTR_F
4kT = 1.6E −20J
at 290_K
Figure 13. 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 noise voltage
contributors. Equation 17 shows the general form for the
output noise voltage using the terms given in Figure 13.
Ǹǒ E_O +
ǒ _{ENI2 ) IBN}
Ǔ R_SǓ² ) 4kTR_S NG² ) ǒI_BI
R_F^Ǔ2 ) 4kTR_FNG
(17)
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 18.
Ǹ ǒ Ǔ EN +
2
ǒ Ǔ ENI2 ) IBN
2
RS ) 4kTRS )
IBI RF
NG
) ^4kTRF
NG
(18)
Evaluating these two equations for the OPA2613 circuit
and component values (see Figure 1) gives a total output
spot noise voltage of 6.34nV/√Hz and a total equivalent
input spot noise voltage of 3.2nV/√Hz. This total input
referred spot noise voltage is higher than the 1.8nV/√Hz
specification for the op amp voltage noise alone. This
reflects the noise added to the output by the inverting
current noise times the feedback resistor.