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OPA743 Datasheet(PDF) 10 Page - Texas Instruments

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Part No. OPA743
Description  12V, 7MHz, CMOS, Rail-to-Rail I/O OPERATIONAL AMPLIFIERS
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Maker  TI1 [Texas Instruments]
Homepage  http://www.ti.com
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OPA743 Datasheet(HTML) 10 Page - Texas Instruments

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OPA743
10
SBOS201
FIGURE 5. OPA743 as Dual Supply Configuration-Buffered References for the DAC7644.
NC
NC
NC
NC
V
OUTA Sense
V
OUTA
V
REFL AB Sense
V
REFL AB
V
REFH AB
V
REFH AB Sense
V
OUTB Sense
V
OUTB
48
47
46
45
44
43
42
41
40
39
38
37
DAC7644
500pF
+V
1/2
OPA2743
1/2
OPA2743
500pF
–V
V
OUT
V
OUT
Ref
+2.5V
V+
Ref
–2.5V
Negative
Reference
Positive
Reference
V–
FIGURE 4. Series Resistor in Unity-Gain Buffer Configura-
tion Improves Capacitive Load Drive.
R
S
20
OPA743
C
L
R
L
V
IN
V
OUT
CAPACITIVE LOAD AND STABILITY
The OPA743 series op amps can drive up to 1000pF pure
capacitive load. Increasing the gain enhances the amplifier’s
ability to drive greater capacitive loads (see the typical
performance curve “Small Signal Overshoot vs Capacitive
Load”).
One method of improving capacitive load drive in the unity-
gain configuration is to insert a 10
Ω to 20Ω resistor inside
the feedback loop, as shown in Figure 4. This reduces
ringing with large capacitive loads while maintaining DC
accuracy.
APPLICATION CIRCUITS
The OPA743 series op amps are optimized for driving
medium-speed sampling data converters. The OPA743 op
amps buffer the converter’s input capacitance and resulting
charge injection while providing signal gain.
Figure 5 shows the OPA743 in a dual supply buffered
reference configuration for the DAC7644.
REFERENCE BUFFER FOR LCD SOURCE DRIVERS
In modern high resolution TFT LCD displays, gamma cor-
rection must be performed to correct for nonlinearities in the
glass transmission characteristics of the LCD panel. The
typical LCD source driver for 64 Bits of Grayscale uses
internal DAC to convert the 6-Bit data into analog voltages
applied to the LCD. These DAC typically require external
voltage references for proper operation. Normally these ex-
ternal reference voltages are generated using a simple resis-
tive ladder, like the one shown in Figure 6.
Typical laptop or desktop LCD panels require 6 to 8 of the
source driver circuits in parallel to drive all columns of the
panel. Although the resistive load of one internal string DAC
is only around 10k
Ω, 6 to 8 in parallel represent a very
substantial load. The power supply used for the LCD source
drivers for laptops is typically in the order of 10V. To
maximize the dynamic range of the DAC, rail-to-rail output
performance is required for the upper and lower buffer. The
OPA4743’s ability to operate on 12V supplies, to drive
heavy resistive loads (as low as 1k
Ω), and to swing to within
325mV of the supply rails, makes it very well suited as a
buffer for the reference voltage inputs of LCD source drivers.
During conversion, the DAC’s internal switches create cur-
rent glitches on the output of the reference buffer. The
capacitor CL (typically 100nF) functions as a charge reser-
voir that provides/absorbs most of the glitch energy. The
series resistor RS isolates the outputs of the OPA4743 from
the heavy capacitive load and helps to improve settling time.


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