OP-184 Datasheet, PDF (14/20 Pages) Analog Devices – Precision Rail-to-Rail Input & Output Operational Amplifiers

OP184/OP284/OP484
A +5 V Only, 12-Bit DAC Swings Rail-to-Rail
The OP284 is ideal for use with a CMOS DAC to generate a
digitally-controlled voltage with a wide output range. Figure 51
shows a DAC8043 used in conjunction with the AD589 to gen-
erate a voltage output from 0 V to 1.23 V. The DAC is actually
operating in “voltage switching” mode where the reference is
connected to the current output, I_OUT, and the output voltage is
taken from the V_REF pin. This topology is inherently noninvert-
ing as opposed to the classic current output mode, which is
inverting and not usable in single supply applications.
+5V
R1
17.8kΩ
1.23V
AD589
8
V_DD
R_FB ²
3
I_OUT
DAC8043 ^VREF ¹
+5V
GND CLK SR1 LD
4765
DIGITAL
CONTROL
3
8
1/2 1
2 ^OP284
4
V_OUT
=
––D––
4096
(5V)
R3
232Ω
1%
R2
32.4kΩ
1%
R4
100kΩ
1%
Figure 51. A +5 V Only, 12-Bit DAC Swings Rail-to-Rail
In this application the OP284 serves two functions. First, it
buffers the high output impedance of the DAC’s V_REF pin,
which is on the order of 10 kΩ. The op amp provides a low
impedance output to drive any following circuitry. Second, the
op amp amplifies the output signal to provide a rail-to-rail out-
put swing. In this particular case, the gain is set to 4.1 so that
the circuit generates a 5 V output when the DAC output is at
full scale. If other output voltage ranges are needed, such as 0 V
≤ V_OUT ≤ 4.095 V, the gain can be easily changed by adjusting
the values of R2 and R3.
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 52 is an example of a +3 V, single-supply high-side cur-
rent monitor that can be incorporated into the design of a volt-
age regulator with fold-back current limiting or a high current
power supply with crowbar protection. This design uses an
OP284’s rail-to-rail input voltage range to sense the voltage
drop across a 0.1 Ω current shunt. A p-channel MOSFET used
as the feedback element in the circuit converts the op amp’s dif-
ferential input voltage into a current. This current is applied to
R2 to generate a voltage that is a linear representation of the
load current. The transfer equation for the current monitor is
given by:
Monitor
Output
=
R2 ×

R_SENSE
R1

×
I_L
For the element values shown, the Monitor Output’s transfer
characteristic is 2.5 V/A.
R_SENSE
0.1Ω
+3V
I_L
+3V
R1
100Ω
S
M1
G
Si9433
MONITOR
OUTPUT
D
R2
2.49kΩ
+3V
0.1µF
3
8
1/2
1
2 ^AD284
4
Figure 52. A High-Side Load Current Monitor
Capacitive Load Drive Capability
The OP284 exhibits excellent capacitive load driving capabili-
ties. It can drive up to 1 nF as shown in Figure 27. Even
though the device is stable, a capacitive load does not come
without penalty in bandwidth. The bandwidth is reduced to
under 1 MHz for loads greater than 2 nF. A “snubber” network
on the output does not increase the bandwidth, but it does sig-
nificantly reduce the amount of overshoot for a given capacitive
load. A snubber consists of a series R-C network (R_S, C_S), as
shown in Figure 53, connected from the output of the device to
ground. This network operates in parallel with the load capaci-
tor, C_L, to provide the necessary phase lag compensation. The
value of the resistor and capacitor is best determined empirically.
V_IN
100mVp-p
+5V
0.1µF
1/2
OP284
R_S
50Ω
C_S
100nF
V_OUT
C_L
1nF
Figure 53. Snubber Network Compensates for Capacitive
Load
The first step is to determine the value of the resistor R_S. A
good starting value is 100 Ω (typically, the optimum value will
be less than 100 Ω). This value is reduced until the small-signal
transient response is optimized. Next, C_S is determined—10 µF
is a good starting point. This value is reduced to the smallest
value for acceptable performance (typically, 1 µF). For the case
of a 10 nF load capacitor on the OP284, the optimal snubber
network is a 20 Ω in series with 1 µF. The benefit is immedi-
ately apparent as shown in the scope photo in Figure 54. The
top trace was taken with a 1 nF load, and the bottom trace was
taken with the 50 Ω, 100 nF snubber network in place. The
amount of overshoot and ringing is dramatically reduced. Table I
below illustrates a few sample snubber networks for large load
capacitors.
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