Datasheet

OPA684
19
SBOS219D
www.ti.com
The total output spot noise voltage can be computed as the
square root of the sum of all squared output noise voltage
contributors. Equation 3 shows the general form for the
output noise voltage using the terms shown in Figure 13.
(3)
E E I R kTR G I R kTR G
O
NI BN
SS
NBIF FN
=+
( )
+




+
( )
+
2
2
2
2
44
Dividing this expression by the noise gain (G
N
= (1 + R
F
/R
G
))
will give the equivalent input referred spot noise voltage at
the non-inverting input, as shown in Equation 4.
(4)
E E I R kTR
IR
G
kTR
G
NNIBN
SS
BI F
N
F
N
=+
( )
++






+
2
2
2
4
4
Evaluating these two equations for the OPA684 circuit and
component values (see Figure 1) will give a total output spot
noise voltage of 13.3nV/√Hz and a total equivalent input spot
noise voltage of 6.7nV/√Hz. This total input referred spot
noise voltage is higher than the 3.7nV/√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. As the gain is increased, this fixed output
noise power term contributes less to the total output noise
and the total input referred voltage noise given by Equation
3 will approach just the 3.7nV/√Hz of the op amp itself. For
example, going to a gain of +20 in the circuit of Figure 1,
adjusting only the gain resistor to 52.3Ω, will give a total input
referred noise of 3.9nV/√Hz . A more complete description of
op amp noise analysis can be found in the TI application note
SBOA066,
Noise Analysis for High Speed Op Amps , located
at www.ti.com.
DC ACCURACY AND OFFSET CONTROL
A current-feedback op amp like the OPA684 provides excep-
tional bandwidth in high gains, giving fast pulse settling but
only moderate DC accuracy. The Electrical Characteristics
show an input offset voltage comparable to high slew rate
voltage-feedback amplifiers. The two input bias currents,
however, are somewhat higher and are unmatched. Whereas
bias current cancellation techniques are very effective with
most voltage-feedback op amps, they do not generally re-
duce 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 1, using worst case +25°C input offset voltage and the
two input bias currents, gives a worst case output offset
range equal to:
±(G
N
• V
OS
) + (I
BN
• R
S
/2 • G
N
) ± (I
BI
• R
F
)
= ±(2 • 3.5mV) ± (10µA • 25Ω • 2) ± (1kΩ • 16µA)
= ±7mV + 0.5mV ± 16mV
= ±23.5mV
where G
N
= non-inverting signal gain
While the last term, the inverting bias current error, is
dominant in this low-gain circuit, the input offset voltage will
become the dominant DC error term as the gain exceeds
5V/V. Where improved DC precision is required in a high-
speed amplifier, consider the OPA642 single and OPA2822
dual voltage-feedback amplifiers.
DISABLE OPERATION
The OPA684 provides an optional disable feature that may
be used to reduce system power when amplifier operation is
not required. If the V
DIS
control pin is left unconnected, the
OPA684 will operate normally. To disable, the V
DIS
control
pin must be asserted LOW. Figure 14 shows a simplified
internal circuit for the disable control feature.
In normal operation, base current to Q1 is provided through
the 250kΩ resistor, while the emitter current through the
40kΩ resistor sets up a voltage drop that is inadequate to
turn on the two diodes in Q1’s emitter. As V
DIS
is pulled LOW,
additional current is pulled through the 40kΩ resistor eventu-
ally turning on these two diodes. At this point, any further
current pulled out of V
DIS
goes through those diodes holding
the emitter-base voltage of Q1 at approximately 0V. This
shuts off the collector current out of Q1, turning the amplifier
off. The supply current in the disable mode is only what is
required to operate the circuit of Figure 14.
25kΩ 250kΩ
40kΩ
I
S
Control
–V
S
+V
S
V
DIS
Q1
When disabled, the output and input nodes go to a high
impedance state. If the OPA684 is operating at a gain of +1
(with a 1kΩ feedback resistor still required for stability), it will
show a very high impedance (1.7pF || 1MΩ) at the output and
exceptional signal isolation. If operating at a gain greater
than +1, the total feedback network resistance (R
F
+ R
G
) will
appear as the impedance looking back into the output, but
the circuit will still show very high forward and reverse
isolation. If configured as an inverting amplifier, the input and
output will be connected through the feedback network
resistance (R
F
+ R
G
) giving relatively poor input to output
isolation.
FIGURE 14. Simplified Disable Control Circuit.