OPA2889 Datasheet, PDF (26/38 Pages) Burr-Brown (TI) – Dual, Low-Power, Wideband, Voltage Feedback OPERATIONAL AMPLIFIER with Disable

OPA2889
SBOS373B – JUNE 2007 – REVISED AUGUST 2008 ....................................................................................................................................................... www.ti.com
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
+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
In normal operation, base current to Q1 is provided
through the 4MΩ resistor, while the emitter current
through the 100kΩ resistor sets up a voltage drop
that is inadequate to turn on the two diodes in the Q1
emitter. As V_DIS is pulled LOW, additional current is
pulled through the 100kΩ resistor, eventually turning
on those two diodes (≈18µA). At this point, any
further current pulled out of V_DIS goes through those
diodes holding the emitter-base voltage of Q1 at
approximately 0V. This current shuts off the collector
current out of Q1, turning the amplifier off. The supply
currents in the disable mode are only those required
to operate the circuit of Figure 63. 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 OPA2889 is operating at
a gain of +1V/V, the device shows a very high
impedance at the output and exceptional signal
isolation. If operating at a gain greater than +1V/V,
the total feedback network resistance (R_F + R_G)
appears as the impedance looking back into the
output, but the circuit still shows very high forward
and reverse isolation. If configured as an inverting
amplifier, the input and output are connected through
the feedback network resistance (R_F + R_G) and the
isolation will be very poor as a result.
THERMAL ANALYSIS
Maximum desired junction temperature sets the
maximum allowed internal power dissipation as
described below. In no case should the maximum
junction temperature be allowed to exceed +150°C.
Operating junction temperature (T_J) is given by T_A +
P_D × θ_JA. The total internal power dissipation (P_D) is
the sum of quiescent power (P_DQ) and additional
power dissipated in the output stage (P_DL) to deliver
load power. Quiescent power is simply the specified
no-load supply current times the total supply voltage
across the part. P_DL depends on the required output
signal and load; for a grounded resistive load, P_DL is
at a maximum when the output is fixed at a voltage
equal to 1/2 of either supply voltage (for equal bipolar
supplies). Under this condition, P_DL = V_S2/(4 × R_L),
where R_L includes feedback network loading.
Note that it is the power in the output stage and not
into the load that determines internal power
dissipation.
150kW
4MW
V_DIS
I_S
Control
-V_S
Figure 63. Simplified Disable Control Circuit
26
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