Datasheet

4kT
R
G
R
G
R
F
R
S
OPA890
I
BI
E
O
I
BN
4kT=1.6E 20J-
at290 K°
E
RS
E
NI
4kTR
S
Ö
4kTR
F
Ö
E
O
+
ǒ
E
2
NI
)
ǒ
I
BN
R
S
Ǔ
2
) 4kTR
S
Ǔ
NG
2
)
(
I
BI
R
F
)
2
) 4kTR
F
NG
Ǹ
E
N
+ E
2
NI
)
ǒ
I
BN
R
S
Ǔ
2
) 4kTR
S
)
ǒ
I
BI
R
F
NG
Ǔ
2
)
4kTR
F
NG
Ǹ
OPA890
www.ti.com SBOS369B –MAY 2007–REVISED DECEMBER 2009
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 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, 1.6μA 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
Figure 52. Op Amp Noise Analysis Model
source resistances appearing at the two inputs. This
matching reduces the output dc error resulting from
The total output spot noise voltage can be computed
the input bias currents to the offset current times the
as the square root of the sum of all squared output
feedback resistor. Evaluating the configuration of
noise voltage contributors. Equation 4 shows the
Figure 46, and using worst-case +25°C input offset
general form for the output noise voltage using the
voltage and current specifications, gives a worst-case
terms shown in Figure 52.
output offset voltage equal to:
±(NG × V
OS(MAX)
) ± (R
F
× I
OS(MAX)
)
= ±(2 × 5mV) ± (750Ω × 0.35μA)
(4)
= ±11.3mV
Dividing this expression by the noise gain [NG = (1 +
R
F
/R
G
)] gives the equivalent input-referred spot noise
with NG = noninverting signal gain
voltage at the noninverting input, as shown in
A fine-scale output offset null or dc operating point
Equation 5.
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
(5)
resistor. In selecting an offset trim method, one key
consideration is the impact on the desired signal path
Evaluating these two equations for the OPA890
frequency response. If the signal path is intended to
circuit and component values (see Figure 46) gives a
be noninverting, the offset control is best applied as
total output spot noise voltage of 17.4nV/√Hz and a
an inverting summing signal to avoid interaction with
total equivalent input spot noise voltage of 8.7nV/√Hz.
the signal source. If the signal path is intended to be
This total includes the noise added by the bias
inverting, applying the offset control to the
current cancellation resistor (175Ω) on the
noninverting input may be considered. However, the
noninverting input. This total input-referred spot noise
dc offset voltage on the summing junction will set up
voltage is only slightly higher than the 8nV/√Hz
a dc current back into the source that must be
specification for the op amp voltage noise alone. This
considered. Applying an offset adjustment to the
result will be the case, as long as the impedances
inverting op amp input can change the noise gain and
appearing at each op amp input are limited to the
frequency response flatness. For a dc-coupled
previously recommend maximum value of 350Ω.
inverting amplifier, see Figure 53 for one example of
Keeping both (R
F
|| R
G
) and the noninverting input
an offset adjustment technique that has minimal
source impedance less than 350Ω satisfies both
impact on the signal frequency response. In this
noise and frequency response flatness
case, the dc offsetting current is brought into the
considerations. Because the resistor-induced noise is
inverting input node through resistor values that are
relatively negligible, additional capacitive decoupling
much larger than the signal path resistors. This
across the bias current cancellation resistor (R
B
) for
configuration ensures that the adjustment circuit has
the inverting op amp configuration of Figure 51 is not
minimal effect on the loop gain and thus, the
required.
frequency response.
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