Datasheet

R
F
750W
±200mVOutputAdjustment
= = 2- -
Power-supply
decouplingnotshown.
5kW
5kW
261W
0.1 Fm
R
G
375W
V
I
20kW
10kW
0.1 Fm
-5V
+5V
1/2
OPA2890
+5V
-5V
V
O
V
O
V
I
R
F
R
G
± ´ ± ´
± ± ´ m
OS(MAX)
(NG V ) (R I )
= (2 5mV) (750 A)
= 11.2mVwith (NG=noninvertingsignalgain)
OS(MAX) F
´ W
± -
1.6
200kW 2MW
50kW
I
S
Control
-V
S
+V
S
V
DIS
Q1
OPA2890
www.ti.com SBOS364C –DECEMBER 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 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:
Figure 61. DC-Coupled, Inverting Gain of –2, with
Offset Adjustment
A fine-scale output offset null, or dc operating point DISABLE OPERATION (MSOP-10 Package
adjustment, is often required. Numerous techniques Only)
are available for introducing dc offset control into an
The OPA2890IDGS provides an optional disable
op amp circuit. Most of these techniques eventually
feature that can be used either to reduce system
reduce to adding a dc current through the feedback
power or to implement a simple channel multiplexing
resistor. In selecting an offset trim method, one key
operation. If the DIS control pin is left unconnected,
consideration is the impact on the desired signal path
the OPA2890IDGS operates normally. To disable, the
frequency response. If the signal path is intended to
control pin must be asserted LOW. Figure 62 shows
be noninverting, the offset control is best applied as
a simplified internal circuit for the disable control
an inverting summing signal to avoid interaction with
feature.
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.
Figure 62. Simplified Disable Control Circuit
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