OPA2890 Datasheet, PDF (25/38 Pages) Texas Instruments – Low-Power, Wideband, Voltage-Feedback OPERATIONAL AMPLIFIER with Disable

OPA2890
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 OPA2890 gives
even tighter control than comparable amplifiers.
Although the high-speed input stage does require
relatively high input bias current (typically 5μA 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
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 49, 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 ´ 1.6mA)
= ±11.2mV 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 61 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
configuration ensures that the adjustment circuit has
minimal effect on the loop gain and thus, the
frequency response.
SBOS364C – DECEMBER 2007 – REVISED DECEMBER 2009
0.1mF
261W
+5V
Power-supply
decoupling not shown.
1/2
OPA2890
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 61. DC-Coupled, Inverting Gain of –2, with
Offset Adjustment
DISABLE OPERATION (MSOP-10 Package
Only)
The OPA2890IDGS 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 OPA2890IDGS operates normally. To disable, the
control pin must be asserted LOW. Figure 62 shows
a simplified internal circuit for the disable control
feature.
+V_S
50kW
Q1
200kW
2MW
V_DIS
I_S
Control
-V_S
Figure 62. Simplified Disable Control Circuit
Copyright © 2007–2009, Texas Instruments Incorporated
Product Folder Link(s): OPA2890
Submit Documentation Feedback
25