OPA2680 Datasheet, PDF (19/21 Pages) Burr-Brown (TI) – Dual Wideband, Voltage Feedback OPERATIONAL AMPLIFIER With Disable

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 add-
ing 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 non-inverting, 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 non-inverting
input may be considered. However, the DC offset voltage on
the summing junction will set up a DC current back into the
source which must be considered. Applying an offset adjust-
ment to the inverting op amp input can change the noise gain
and frequency response flatness. For a DC-coupled invert-
ing amplifier, Figure 13 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 will
insure that the adjustment circuit has minimal effect on the
loop gain and hence the frequency response.
0.1µF
328Ω
+5V
Supply Decoupling
Not Shown
1/2
OPA2680
V_O
+5V
5kΩ
10kΩ
5kΩ
R_G
500Ω
V_I
20kΩ
0.1µF
–5V
–5V
R_F
1kΩ
±200mV Output Adjustment
V_{O = –} R_{F = –2}
V_I
R_G
FIGURE 13. DC-Coupled, Inverting Gain of –2, with Offset
Adjustment.
+V_S
15kΩ
Q1
25kΩ
110kΩ
V_DIS
I_S
Control
–V_S
FIGURE 14. Simplified Disable Control Circuit.
eventually turning on those two diodes (≈100uA). At this
point, any further current pulled out of V_DIS goes through
those diodes holding the emitter-base voltage of Q1 at
approximately zero volts. This shuts off the collector current
out of Q1, turning the amplifier off. The supply current in
the disable mode are only those required to operate the
circuit of Figure 14. Additional circuitry ensures that turn-on
time occurs faster than turn-off time (make-before-break).
When disabled, the output and input nodes go to a high
impedance state. If the OPA2680 is operating in a gain of
+1, this will show a very high impedance at the output and
exceptional signal isolation. If operating at a gain greater
than +1, the total feedback network resistance (R_F + R_G) will
appear as the impedance looking back into the output, but
the circuit will still show very high forward and reverse
isolation. If configured as an inverting amplifier, the input
and output will be connected through the feedback network
resistance (R_F + R_G) and the isolation will be very poor as a
result.
One key parameter in disable operation is the output glitch
when switching in and out of the disabled mode. Figure 15
shows these glitches for the circuit of Figure 1 with the input
signal at 0V. The glitch waveform at the output pin is plotted
along with the DIS pin voltage.
DISABLE OPERATION (SO-14 Package Only)
The OPA2680N provides an optional disable feature that
40
may be used either to reduce system power or to implement
20
a simple channel multiplexing operation. If the DIS control
0
pin is left unconnected, the OPA2680N will operate nor-
–20
mally. To disable, the control pin must be asserted LOW.
–40
Figure 14 shows a simplified internal circuit for the disable
control feature.
Output Voltage
(0V Input)
V_DIS
4.8V
In normal operation, base current to Q1 is provided through
0.2V
the 110kΩ resistor, while the emitter current through the
15kΩ resistor sets up a voltage drop that is inadequate to
turn on the two diodes in Q1’s emitter. As V_DIS is pulled
LOW, additional current is pulled through the 15kΩ resistor
Time (20ns/div)
FIGURE 15. Disable/Enable Glitch.
®
19
OPA2680