OP296 Analog Devices, OP296 Datasheet - Page 14

• Manufacturer Part Number: OP296
• Description: Micropower, RRIO Dual Op Amp
• Manufacturer: Analog Devices

Specifications of OP296

• -3db Bandwidth: 450kHz
• Slew Rate: 300mV/µs
• Vos: 35µV
• Ib: 10nA
• # Opamps Per Pkg: 2
• Input Noise (nv/rthz): 26nV/rtHz
• Vcc-vee: 3V to 12V
• Isy Per Amplifier: 60µA
• Packages: SOIC,SOP

TrimDAC is a registered trademark of Analog Devices Inc.
OP196/OP296/OP496
Figure 9 shows the regulator’s recovery characteristics when its
output underwent a 20 mA to 50 mA step current change.
Buffering a DAC Output
Multichannel TrimDACs ® such as the AD8801/AD8803, are
widely used for digital nulling and similar applications. These
DACs have rail-to-rail output swings, with a nominal output
resistance of 5 kΩ. If a lower output impedance is required, an
OP296 amplifier can be added. Two examples are shown in
Figure 10. One amplifier of an OP296 is used as a simple buffer
to reduce the output resistance of DAC A. The OP296 provides
rail-to-rail output drive while operating down to a 3 V supply
and requiring only 50 µA of supply current.
5V TO 3.2V
V
REFL
V
WAVEFORM
REFH
CURRENT
CONTROL
V
AD8801/
AD8803
V
V
V
V
V
V
IN
L
L
L
H
H
H
GND
5V
V
STEP
DD
OUTPUT
DIGITAL INTERFACING
OMITTED FOR CLARITY
50mA
30mA
MJE 350
100
0%
90
10
1000pF
R1
100k
1
10mV
2V
OP296
43k
8
4
1/2
OP296
3
2
1.235V
AD589
44.2k
1%
30.9k
1%
SUMMER CIRCUIT
WITH FINE TRIM
ADJUSTMENT
SIMPLE BUFFER
0V TO 5V
+4.983V
+1.1mV
50µs
I
L
<
100 F
50mA
V
O
The next two DACs, B and C, sum their outputs into the other
OP296 amplifier. In this circuit DAC C provides the coarse
output voltage setting and DAC B is used for fine adjustment.
The insertion of R1 in series with DAC B attenuates its contri-
bution to the voltage sum node at the DAC C output.
A High-Side Current Monitor
In the design of power supply control circuits, a great deal of
design effort is focused on ensuring a pass transistor’s long-term
reliability over a wide range of load current conditions. As a result,
monitoring and limiting device power dissipation is of prime
importance in these designs. The circuit illustrated in Figure 11
is an example of a 5 V, single-supply high-side current monitor
that can be incorporated into the design of a voltage regulator
with fold-back current limiting or a high current power supply
with crowbar protection. This design uses an OP296’s rail-to-
rail input voltage range to sense the voltage drop across a 0.1 Ω
current shunt. A p-channel MOSFET is used as the feedback
element in the circuit to convert the op amp’s differential input
voltage into a current. This current is then applied to R2 to gen-
erate a voltage that is a linear representation of the load current.
The transfer equation for the current monitor is given by:
For the element values shown, the Monitor Output’s transfer
characteristic is 2.5 V/A.
A Single-Supply RTD Amplifier
The circuit in Figure 12 uses three op amps on the OP496 to
produce a bridge driver for an RTD amplifier while operating
from a single 5 V supply. The circuit takes advantage of the
OP496’s wide output swing to generate a bridge excitation
voltage of 3.9 V. An AD589 provides a 1.235 V reference for
the bridge current. Op amp A1 drives the bridge to maintain
1.235 V across the parallel combination of the 6.19 kΩ and
2.55 MΩ resistors, which generates a 200 µA current source.
This current divides evenly and flows through both halves of
the bridge. Thus, 100 µA flows through the RTD to generate
an output voltage which is proportional to its resistance. For
improved accuracy, a 3-wire RTD is recommended to balance
the line resistance in both 100 Ω legs of the bridge.
MONITOR
OUTPUT
Monitor Output = R2 ×
5V
2.49k
3N163
100
R1
M1
R2
S
D
R
0.1
SENSE
G



R
3
2
SENSE
OP296
R1
1/2
I
L
5V
8
4

 × I

1
L
5V
E