OPA890IDBV BURR-BROWN [Burr-Brown Corporation], OPA890IDBV Datasheet - Page 22

• Manufacturer Part Number: OPA890IDBV
• Description: Low-Power, Wideband, Voltage-Feedback OPERATIONAL AMPLIFIER with Disable
• Manufacturer: BURR-BROWN [Burr-Brown Corporation]

OPA890
SBOS369 – MAY 2007
22
The total output spot noise voltage can be computed
as the square root of the sum of all squared output
noise voltage contributors.
general form for the output noise voltage using the
terms shown in
E
Dividing this expression by the noise gain [NG = (1 +
R
voltage at the noninverting input, as shown in
Equation
E
Evaluating these two equations for the OPA890
circuit and component values (see
total output spot noise voltage of 17.4nV/ Hz and a
total
8.7nV/ Hz. This total includes the noise added by
the bias current cancellation resistor (175 ) on the
noninverting input. This total input-referred spot
noise voltage is only slightly higher than the 8nV/ Hz
specification for the op amp voltage noise alone.
This result will be the case, as long as the
impedances appearing at each op amp input are
limited to the previously recommend maximum value
of
noninverting input source impedance less than 350
satisfies both noise and frequency response flatness
considerations. Because the resistor-induced noise is
relatively negligible, additional capacitive decoupling
across the bias current cancellation resistor (R
the inverting op amp configuration of
required.
O
F
N
E
/R
+
RS
+
R
350 .
S
Figure 52. Op Amp Noise Analysis Model
G
)] gives the equivalent input-referred spot noise
equivalent
E
Ö
5.
E
2
NI
4kTR
2
NI
) I
4kT
Keeping
) I
R
S
G
BN
I
Figure
BN
R
BN
S
R
2
input
S
) 4kTR
E
NI
2
52.
) 4kTR
both
R
G
spot
S
OPA890
Equation 4
NG
(R
S
2
)
F
I
) ( I
BI
noise
R
Figure
F
4kT = 1.6E
I
BI
NG
BI
Figure 51
R
R
Ö
R
at 290 K
G
4kTR
F
)
F
)
2
voltage
) 4kTR
46) gives a
shows the
2
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F
°
)
and
-
20J
4kTR
is not
B
NG
) for
F
E
NG
the
O
(4)
(5)
of
F
DC ACCURACY AND OFFSET CONTROL
The
voltage-feedback op amp allows good output dc
accuracy in a wide variety of applications. The
power-supply current trim for the OPA890 gives even
tighter control than comparable amplifiers. Although
the high-speed input stage does require relatively
high input bias current (+25 C worst case, 1.6 A 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
matching reduces the output dc error resulting from
the input bias currents to the offset current times the
feedback resistor. Evaluating the configuration of
Figure
voltage
worst-case output offset voltage equal to:
= (2
= 11.3mV
with NG = noninverting signal gain
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
op amp circuit. Most of these techniques eventually
reduce to adding a dc current through the feedback
resistor. In selecting an offset trim method, one key
consideration is the impact on the desired signal
path 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 inverting, applying the offset control to
the noninverting input may be considered. However,
the dc offset voltage on the summing junction will set
up a dc current back into the source that must be
considered. Applying an offset adjustment to the
inverting op amp input can change the noise gain
and frequency response flatness. For a dc-coupled
inverting amplifier, see
an offset adjustment technique that has minimal
impact on the signal frequency response. In this
case, the dc offsetting current is brought into the
inverting input node through resistor values that are
much larger than the signal path resistors. This
configuration ensures that the adjustment circuit has
minimal effect on the loop gain and thus, the
frequency response.
(NG
balanced
46, and using worst-case +25 C input offset
5mV)
V
OS(MAX)
and
(750
)
current
input
(R
F
Figure 53
0.35 A)
I
OS(MAX)
stage
specifications,
)
for one example of
of
a
www.ti.com
wideband
gives
a