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
±200mVOutputAdjustment
= = 2- -
Power-supply
decouplingnotshown.
5kW
5kW
328W
0.1 Fm
R
G
500W
V
I
20kW
10kW
0.1 Fm
-5V
+5V
1/2
OPA2690
+5V
-5V
V
O
V
O
V
I
R
F
R
G
OPA2690
www.ti.com SBOS238G –JUNE 2002–REVISED MARCH 2010
The total output spot noise voltage can be computed ±(NG × V
OS(MAX)
) ± (R
F
× I
OS(MAX)
)
as the square root of the sum of all squared output
= ±(2 × 4.5mV) ± (402Ω × 1mA)
noise voltage contributors. Equation 7 shows the
= ±9.4mV – (NG = noninverting signal gain)
general form for the output noise voltage using the
A fine-scale output offset null, or dc operating point
terms shown in Figure 50.
adjustment, is often required. Numerous techniques
are available for introducing dc offset control into an
op amp circuit. Most of these techniques eventually
(7)
reduce to adding a dc current through the feedback
Dividing this expression by the noise gain [NG = (1 +
resistor. In selecting an offset trim method, one key
R
F
/R
G
)] will give the equivalent input-referred spot
consideration is the impact on the desired signal path
noise voltage at the noninverting input, as shown in
frequency response. If the signal path is intended to
Equation 8.
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
inverting, applying the offset control to the
(8)
noninverting input may be considered. However, the
dc offset voltage on the summing junction will set up
Evaluating these two equations for the OPA2690
a dc current back into the source that must be
circuit and component values (see Figure 36) gives a
considered. Applying an offset adjustment to the
total output spot noise voltage of 12.3nV/√Hz and a
inverting op amp input can change the noise gain and
total equivalent input spot noise voltage of 6.1nV/√Hz.
frequency response flatness. For a dc-coupled
This is including the noise added by the bias current
inverting amplifier, Figure 51 shows one example of
cancellation resistor (175Ω) on the noninverting input.
an offset adjustment technique that has minimal
This total input-referred spot noise voltage is only
impact on the signal frequency response. In this
slightly higher than the 5.5nV/√Hz specification for the
case, the dc offsetting current is brought into the
op amp voltage noise alone. This will be the case as
inverting input node through resistor values that are
long as the impedances appearing at each op amp
much larger than the signal path resistors. This
input are limited to the previously recommend
ensures that the adjustment circuit has minimal effect
maximum value of 300Ω. Keeping both (R
F
|| R
G
) and
on the loop gain and hence, the frequency response.
the noninverting input source impedance less than
300Ω will satisfy both noise and frequency response
flatness considerations. As 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 47 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 OPA2690 gives
even tighter control than comparable amplifiers.
Although the high-speed input stage does require
relatively high input bias current (typically 5mA out of
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
Figure 51. DC-Coupled, Inverting Gain of -2, with
currents to the offset current times the feedback
Offset Adjustment
resistor. 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:
BLANKSPACE
BLANKSPACE
Copyright © 2002–2010, Texas Instruments Incorporated 25
Product Folder Link(s): OPA2690