OPA2690 Datasheet, PDF (23/30 Pages) Burr-Brown (TI) – Dual, Wideband, Voltage-Feedback OPERATIONAL AMPLIFIER with Disable

OPA2690
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
OPA2690 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 reduces the
output DC error due to the input bias currents to the offset
current times the feedback resistor. Evaluating the
configuration of Figure 1, 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 × 4.5mV) ± (402Ω × 1µA)
= ±9.4mV − (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, Figure 16
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.
SBOS238D − JUNE 2002 − REVISED DECEMBER 2004
0.1µF
328Ω
+5V
Power−supply
decoupling not shown.
1/2
OPA2690
V_O
+5V
5kΩ ^VI
R_G
500Ω
10kΩ
5kΩ
20kΩ
0.1µF
−5V
−5V
R_F
1kΩ
±200mV Output Adjustment
V_{O = −} R_{F = −2}
V_I
R_G
Figure 16. DC-Coupled, Inverting Gain of −2, with
Offset Adjustment
DISABLE OPERATION (SO-14 Package Only)
The OPA2690I-14D 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 OPA2690I-14D will
operate normally. To disable, the control pin must be
asserted LOW. Figure 17 shows a simplified internal
circuit for the disable control feature.
+V_S
15kΩ
Q1
25kΩ
110kΩ
V_DIS
I_S
Control
−V_S
Figure 17. Simplified Disable Control Circuit
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