OPA2889 Datasheet, PDF (25/35 Pages) Burr-Brown (TI) – Dual, Low-Power, Wideband, Voltage Feedback OPERATIONAL AMPLIFIER with Disable

OPA2889
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considerations. Because 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 60 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 OPA2889 gives
even tighter control than comparable amplifiers.
Although the high-speed input stage does require
relatively low 0.75μA input bias current, 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 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 50, and using worst-case
+25°C input offset voltage and current specifications,
gives a worst-case output offset voltage equal to:
±(NG ´ V_OS(MAX)) ± (R_F ´ I_OS(MAX))
= ±(2 ´ 5mV) ± (750W ´ 0.75mA)
= ±10.6mV 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 sets
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, Figure 62 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
technique ensures that the adjustment circuit has
minimal effect on the loop gain and thus, the
frequency response.
0.1mF
250W
SBOS373 – JUNE 2007
+5V
Power-supply
decoupling not shown.
1/2
OPA2889
V_O
+5V
V_I
5kW
R_G
375W
10kW
5kW
20kW
0.1mF
-5V
-5V
R_F
750W
±200mV Output Adjustment
V_{O = -} R_{F = -2}
V_I
R_G
Figure 62. DC-Coupled, Inverting Gain of –2V/V,
with Offset Adjustment
DISABLE OPERATION (MSOP-10 Package
Only)
The OPA2889IDGS provides an optional disable
feature that can be used either to reduce system
power or to implement a simple channel multiplexing
operation. If the DIS control pin is left unconnected,
the OPA2889IDGS operates normally. To disable,
the control pin must be asserted LOW. Figure 63
shows a simplified internal circuit for the disable
control feature.
+V_S
100kW
Q1
150kW
4MW
V_DIS
I_S
Control
-V_S
Figure 63. Simplified Disable Control Circuit
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