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參數(shù)資料
| 型號： | AD8131ARM-REEL |
| 廠商： | ANALOG DEVICES INC |
| 元件分類： | 運(yùn)動控制電子 |
| 英文描述： | Low-Cost, High-Speed Differential Driver |
| 中文描述： | OP-AMP, 7000 uV OFFSET-MAX, PDSO8 |
| 封裝： | MO-187AA, MSOP-8 |
| 文件頁數(shù)： | 10/12頁 |
| 文件大小： | 197K |
| 代理商： | AD8131ARM-REEL |
REV. 0
AD8131
–10–
R
R
R
R
R
k
IN dm
,
G
F
G
F
.
=
×
+
(
)
=
1
2
1 125
The circuit’s input impedance is effectively higher than it would
be for a conventional op amp connected as an inverter because
a fraction of the differential output voltage appears at the inputs
as a common-mode signal, partially bootstrapping the voltage
across the input resistor R
G
.
Input Common-Mode Voltage Range in Single Supply
Applications
The AD8131 is optimized for level-shifting “ground” referenced
input signals. For a single-ended input this would imply, for
example, that the voltage at –D
IN
in Figure 37 would be zero
volts when the amplifier’s negative power supply voltage (at V–)
was also set to zero volts.
Setting the Output Common-Mode Voltage
The AD8131’s V
OCM
pin is internally biased at a voltage
approximately equal to the midsupply point (average value of
the voltages on V+ and V–). Relying on this internal bias will
result in an output common-mode voltage that is within about
25 mV of the expected value.
In cases where more accurate control of the output common-
mode level is required, it is recommended that an external
source, or resistor divider (made up of 10 k
resistors), be used.
Driving a Capacitive Load
A purely capacitive load can react with the pin and bondwire
inductance of the AD8131 resulting in high frequency ringing in
the pulse response. One way to minimize this effect is to place a
small resistor in series with the amplifier’s outputs as shown in
Figure 23.
APPLICATIONS
Twisted-Pair Line Driver
The AD8131 has on-chip resistors that provide for a gain-of-
two without any external parts. Several on-chip resistors are
trimmed to ensure that the gain is accurate, the common-mode
rejection is good, and the output is well balanced. This makes
the AD8131 very suitable as a single-ended-to-differential
twisted-pair line driver.
Figure 38 shows a circuit of an AD8131 driving a twisted-pair
line, like a Category 3 or Category 5 (Cat3 or Cat5), that are
already installed in many buildings for telephony and data com-
munications. The characteristic impedance of such transmission
lines is usually about 100
. The outstanding balance of the
AD8131 output will minimize the common-mode signal and there-
fore the amount of EMI generated by driving the twisted pair.
The two resistors in series with each output terminate the line at
the transmit end. Since the impedances of the outputs of the
AD8131 are very low, they can be thought of as a short circuit,
and the two terminating resistors form a 100
termination at
the transmit end of the transmission line. The receive end is
directly terminated by a 100
resistor across the line.
This back-termination of the transmission line divides the out-
put signal by two. The fixed gain of two of the AD8131 will
create a net unity gain for the system from end to end.
In this case, the input signal is provided by a signal generator
with an output impedance of 50
. This is terminated with a
49.9
resistor near +D
IN
of the AD8131. The effective parallel
resistance of the source and termination is 25
. The 24.9
resistor from –D
IN
to ground matches the +D
IN
source impedance
and minimizes any dc and gain errors.
If +D
IN
is driven by a low-impedance source over a short dis-
tance, such as the output of an op amp, then no termination
resistor is required at +D
IN
. In this case, the –D
IN
can be
directly tied to ground.
+3 V Supply Differential A-to-D Driver
Many newer A-to-D converters can run from a single +3 V
supply, which can save significant system power. In order to
increase the dynamic range at the analog input, they have differ-
ential inputs, which doubles the dynamic range with respect to a
single-ended input. An added benefit of using a differential
input is that the distortion can be improved.
The low distortion and ability to run from a single +3 V supply
make the AD8131 suited as an A-to-D driver for some 10-bit,
single supply applications. Figure 39 shows a schematic for a
circuit for an AD8131 driving an AD9203, a 10-bit, 40 MSPS
A-to-D converter.
The common mode of the AD8131 output is set at midsupply
by the voltage divider connected to V
OCM
, and ac bypassed with
a 0.1
μ
F capacitor. This provides for maximum dynamic range
between the supplies at the output of the AD8131. The 110
resistors at the AD8131 output, along with the shunt capacitors
form a one pole, low-pass filter for lowering noise and antialiasing.
Figure 40 shows an FFT plot that was taken from the combined
devices at an analog input frequency of 2.5 MHz and a 40 MSPS
sampling rate. The performance of the AD8131 compares very
favorably with a center-tapped transformer drive, which has
typically been the best way to drive this A-to-D converter. The
AD8131 has the advantage of maintaining dc performance,
which a transformer solution cannot provide.
Unity-Gain, Single-Ended-to-Differential Driver
If it is not necessary to offset the output common-mode volt-
age (via the V
OCM
pin), then the AD8131 can make a simple
unity-gain single-ended-to-differential amplifier that does not
require any external components. Figure 41 shows the schematic
for this circuit.
Referring to Figure 2, when –D
IN
is left floating, there is 100
percent feedback of +OUT to –IN via the internal feedback
resistor. This contrasts with the typical gain-of-two operation
where –D
IN
is grounded and one third of the +OUT is fed back
to –IN. The result is a closed-loop differential gain of one.
Upon careful observation, it can be seen that only +D
IN
and
V
OCM
are referenced to ground. It is the case that the ground
voltage at V
OCM
is the reference for this circuit. In this unity
gain configuration, if a dc voltage is applied to V
OCM
to shift the
common-mode voltage, a differential dc voltage will be created
at the output, along with the common-mode voltage change.
Thus, this configuration cannot be used when it is desired to
offset the common-mode voltage of the output with respect to
the input at +D
IN
.
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