Datasheet
E =
O
E
NI
2
+(I R ) +4kTR
BN S S
2
NG +(I R ) +4kTR NG
2 2
BI F F
E =
N
I R
BI F
NG
E +(I R ) +4kTR +
NI BN S S
22
+
4kTR
F
NG
2
R
F
1kW
±200mVOutputAdjustment
= = 2- -
Power-supplydecoupling
notshown.
5kW
5kW
328W
0.1 Fm
R
G
500W
V
I
20kW
10kW
0.1 Fm
-5V
+5V
OPA690
+5V
-5V
V
O
V
O
V
I
R
F
R
G
OPA690
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SBOS223F –DECEMBER 2001–REVISED FEBRUARY 2010
The total output spot noise voltage can be computed –(NG = noninverting signal gain)
as the square root of the sum of all squared output
±(NG × V
OS(MAX)
) ± (R
F
× I
OS(MAX)
)
noise voltage contributors. Equation 3 shows the
= ±(2 × 4mV) ± (402Ω × 1mA)
general form for the output noise voltage using the
= ±8.4mV
terms shown in Figure 46.
A fine-scale output offset null, or dc operating point
adjustment, is often required. Numerous techniques
are available for introducing dc offset control into an
(3)
op amp circuit. Most of these techniques eventually
Dividing this expression by the noise gain [NG = (1 +
reduce to adding a dc current through the feedback
R
F
/R
G
)] will give the equivalent input-referred spot
resistor. In selecting an offset trim method, one key
noise voltage at the noninverting input, as shown in
consideration is the impact on the desired signal path
Equation 4.
frequency response. If the signal path is intended to
be noninverting, the offset control is best applied as
an inverting summing signal to avoid interaction with
the signal source. If the signal path is intended to be
(4)
inverting, applying the offset control to the
noninverting input may be considered. However, the
Evaluating these two equations for the OPA690
dc offset voltage on the summing junction will set up
circuit and component values (see Figure 36) gives a
a dc current back into the source that must be
total output spot noise voltage of 12.3nV/√Hz and a
considered. Applying an offset adjustment to the
total equivalent input spot noise voltage of 6.1nV/√Hz.
inverting op amp input can change the noise gain and
This is including the noise added by the bias current
frequency response flatness. For a dc-coupled
cancellation resistor (175Ω) on the noninverting input.
inverting amplifier, see Figure 47 for one example of
This total input-referred spot noise voltage is only
an offset adjustment technique that has minimal
slightly higher than the 5.5nV/√Hz specification for the
impact on the signal frequency response. In this
op amp voltage noise alone. This will be the case as
case, the dc offsetting current is brought into the
long as the impedances appearing at each op amp
inverting input node through resistor values that are
input are limited to the previously recommend
much larger than the signal path resistors. This
maximum value of 300Ω. Keeping both (R
F
|| R
G
) and
ensures that the adjustment circuit has minimal effect
the noninverting input source impedance less than
on the loop gain and hence, the frequency response.
300Ω will satisfy both noise and frequency response
flatness considerations. Since the resistor-induced
noise is relatively negligible, additional capacitive
decoupling across the bias current cancellation
resistor (R
B
) for the inverting op amp configuration of
Figure 43 is not required.
DC ACCURACY AND OFFSET CONTROL
The balanced input stage of a wideband
voltage-feedback op amp allows good output dc
accuracy in a wide variety of applications. The
power-supply current trim for the OPA690 gives even
tighter control than comparable amplifiers. Although
the high-speed input stage does require relatively
high input bias current (typically ±8µA at each input
terminal), the close matching between them may be
used to reduce the output dc error caused by this
current. The total output offset voltage may be
considerably reduced by matching the dc source
resistances appearing at the two inputs. This reduces
the output dc error due to the input bias currents to
Figure 47. DC-Coupled, Inverting Gain of -2, with
the offset current times the feedback resistor.
Offset Adjustment
Evaluating the configuration of Figure 36, and using
worst-case +25°C input offset voltage and current
specifications, gives a worst-case output offset
voltage equal to:
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