OPA3690ID Datasheet, PDF (25/39 Pages) Texas Instruments – Triple, Wideband, Voltage-Feedback OPERATIONAL AMPLIFIER with Disable

OPA3690
www.ti.com
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 OPA3690 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
currents to the offset current times the feedback
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:
– (NG = noninverting signal gain)
±(NG × V_OS(MAX)) ± (R_F × I_OS(MAX))
= ±(2 × 4.5mV) ± (250Ω × 1mA)
= ±9.25mV
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
SBOS237G – MARCH 2002 – REVISED MARCH 2010
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, Figure 51 shows one example of
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
ensures that the adjustment circuit has minimal effect
on the loop gain and hence, the frequency response.
0.1mF
328W
+5V
Power-supply
decoupling not shown.
1/3
OPA3690
V_O
+5V
V_I
5kW
R_G
200W
10kW
5kW
1.25kW
0.1mF
-5V
-5V
R_F
402W
±200mV Output Adjustment
V_{O = -} R_{F = -2}
V_I
R_G
Figure 51. DC-Coupled, Inverting Gain of –2, with
Offset Adjustment
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