OPA3693 Datasheet, PDF (24/29 Pages) Burr-Brown (TI) – Triple, Ultra-Wideband, Fixed-Gain, VIDEO BUFFER with Disable

OPA3693
SBOS353 – DECEMBER 2006
The total output spot noise voltage can be computed
as the square root of the sum of all squared output
noise voltage contributors. Equation 1 shows the
general form for the output noise voltage using the
terms shown in Figure 57.
E_O =
E_NI² + (I_BNR_S)² + 4kTR_S NG² + (I_BIR_F)² + 4kTR_FNG
Dividing this expression through by noise gain (NG =
1 + R_F/R_G) gives the equivalent input-referred spot
noise voltage at the noninverting input, as shown in
Equation 2.
E_N =
E_NI² + (I_BNR_S)² + 4kTR_S +
I_BIR_F
NG
2
+
4kTR_F
NG
Evaluating the output noise and input noise
expressions for the two noninverting gain
configurations, and with two different values for the
noninverting source impedance, gives output and
input-referred spot noise voltages of Table 3.
Table 3. Total Output and Input-Referred Noise
CONFIGURATION
G = +2 (Figure 42)
G = +2 (Figure 42)
G = +1 (Figure 43)
G = +1 (Figure 43)
OUTPUT SPOT TOTAL INPUT
R_S
NOISE
SPOT NOISE
(Ω)
E_O (nV/√Hz)
E_N (nV/√Hz)
25
8.3
4.15
300
14
7
25
7.3
7.3
300
9.2
9.2
The output noise is being dominated by the inverting
current noise times the internal feedback resistor.
This gives a total input-referred noise voltage that
exceeds the 1.8nV voltage term for the amplifier
itself.
DC ACCURACY AND OFFSET CONTROL
A current-feedback op amp such as the OPA3693
provides exceptional bandwidth and slew rate 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. Whereas 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. Since the two input bias
currents are unrelated in both magnitude and
polarity, matching the source impedance looking out
of each input to reduce their error contribution to the
output is ineffective. Evaluating the configuration of
Figure 42, using worst-case +25°C input offset
voltage and the two input bias currents, gives a
worst-case output offset range equal to:
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±(NG ´ V_OS) + (I_BN ´ R_S/2 ´ NG) ± (I_BI ´ R_F)
= ±(2 ´ 3.5mV) + (35mA ´ 25W ´ 2) ± (50mA ´ 300W)
= ±7mV ± 1.75mV ± 15mV
= ±23.75mV
where NG = noninverting signal gain.
Minimizing the resistance seen by the noninverting
input also minimizes the output dc error. For
improved dc precision in a wideband low-gain
amplifier, consider the OPA842 where a bipolar input
is acceptable (low source resistance) or the OPA656
where a JFET input is required.
DISABLE OPERATION
The OPA3693 provides an optional disable feature
that can be used to reduce system power. If the V_DIS
control pin is left unconnected, the OPA3693
operates normally. This shutdown is intended only as
a power-savings feature. Forward path isolation
when disabled is very good for small signals for
gains of +1 or +2. Large-signal isolation is not
ensured. Using this feature to multiplex two or more
outputs together is not recommended. Large signals
applied to the disabled output stages can turn on
parasitic devices degrading signal linearity for the
desired channel.
Turn-on time is very quick from the shutdown
condition (typically < 60ns). Turn-off time strongly
depends on the selected gain configuration and load,
but is typically 3µs for the circuit of Figure 42.
To shutdown, the control pin must be asserted low.
This logic control is referenced to the positive supply,
as the simplified circuit of Figure 58 shows.
+V_S
15kW
Q1
V_DIS
25kW
I_S
Control
110kW
-V_S
Figure 58. Simplified Disable Control Circuit
In normal operation, base current to Q1 is provided
through the 110kΩ resistor while the emitter current
through the 15kΩ resistor sets up a voltage drop that
is inadequate to turn on the two diodes in the Q1
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