OMO ElectronicOMO Electronic
Expand menu
Hello, Sign inMy Account
0 Cart

LT1969CMS#PBF

P1-P3P4-P6P7-P9P10-P12P13-P15P16-P18P19-P20
13
LT1969
network is bootstrapped. This network can also be placed
between the inverting input and an AC ground.
Another compensation scheme for noninverting circuits is
shown in Figure 4. The circuit is unity gain at low frequency
and a gain of 1 + R
F
/R
G
at high frequency. The DC output
offset is reduced by a factor of ten. The techniques of
Figures 3 and 4 can be combined as shown in Figure 5. The
gain is unity at low frequencies, 1 + R
F
/R
G
at mid-band and
for stability, a gain of 10 or greater at high frequencies.
Output Loading
The LT1969 output stage is very wide bandwidth and able
to source and sink large currents. Reactive loading, even
isolated with a back-termination resistor, can cause ring-
ing at frequencies of hundreds of MHz. For this reason, any
design should be evaluated over a wide range of output
conditions. To reduce the effects of reactive loading, an
optional snubber network consisting of a series RC across
the load can provide a resistive load at high frequency.
Another option is to filter the drive to the load. If a back-
termination resistor is used, a capacitor to ground at the
load can eliminate ringing.
Line Driving Back-Termination
The standard method of cable or line back-termination is
shown in Figure 6. The cable/line is terminated in its
characteristic impedance (50Ω, 75Ω, 100Ω, 135Ω, etc.).
A back-termination resistor also equal to to the
chararacteristic impedance should be used for maximum
pulse fidelity of outgoing signals, and to terminate the line
for incoming signals in a full-duplex application. There are
three main drawbacks to this approach. First, the power
dissipated in the load and back-termination resistors is
equal so half of the power delivered by the amplifier is
Figure 5. Combination Compensation
R
C
V
o
V
i
C
C
+
–
1969 F05
R
F
R
G
C
BIG
R
F
R
G
= 1 AT LOW FREQUENCIES
= 1 + AT MEDIUM FREQUENCIES
R
F
(R
C
|| R
G
)
= 1 + AT HIGH FREQUENCIES
V
o
V
i
Figure 4. Alternate Noninverting Compensation
+
–
1969 F04
R
F
R
G
V
i
V
O
C
C
< 15MHz
1
2πR
G
C
C
R
G
≤ R
F
/9
= 1 (LOW FREQUENCIES)
(HIGH FREQUENCIES)
V
o
V
i
= 1 +
R
F
R
G
Figure 6. Standard Cable/Line Back-Termination
+
–
1969 F06
R
F
R
BT
CABLE OR LINE WITH
CHARACTERISTIC IMPEDANCE R
L
R
G
V
O
V
i
R
L
(1 + R
F
/R
G
)
=
V
o
V
i
1
2
R
BT
= R
L
APPLICATIO S I FOR ATIO
WUUU
14
LT1969
APPLICATIO S I FOR ATIO
WUUU
wasted in the termination resistor. Second, the signal is
halved so the gain of the amplifer must be doubled to have
the same overall gain to the load. The increase in gain
increases noise and decreases bandwidth (which can also
increase distortion). Third, the output swing of the ampli-
fier is doubled which can limit the power it can deliver to
the load for a given power supply voltage.
An alternate method of back-termination is shown in
Figure 7. Positive feedback increases the effective back-
termination resistance so R
BT
can be reduced by a factor
of n. To analyze this circuit, first ground the input. As R
BT
␣=
R
L
/n, and assuming R
P2
>>R
L
we require that:
V
a
= V
o
(1 – 1/n) to increase the effective value of
R
BT
by n.
V
p
= V
o
(1 – 1/n)/(1 + R
F
/R
G
)
V
o
= V
p
(1 + R
P2
/R
P1
)
Figure 7. Back-Termination Using Positive Feedback
+
–
1969 F07
R
F
R
BT
R
P2
R
P1
R
G
V
i
V
a
V
P
V
o
R
L
R
F
R
G
1 +
R
L
n
=
V
o
V
i
= 1 –
–
1
n
FOR R
BT
=
()
R
F
R
G
1 +
()
R
P1
R
P1
+ R
P2
R
P1
R
P2
+ R
P1
R
P2
/(R
P2
+ R
P1
)
()
1 + 1/n
Eliminating Vp, we get the following:
(1 + R
P2
/R
P1
) = (1 + R
F
/R
G
)/(1 – 1/n)
For example, reducing R
BT
by a factor of n = 4, and with an
amplifer gain of (1 + R
F
/R
G
) = 10 requires that R
P2
/R
P1
=␣ 12.3.
Note that the overall gain is increased:
V
V
RRR
nRRRRR
o
i
PPP
FG P P P
=
+
()
+
()
+
()
[]
−+
()
[]
221
121
11 1
/
// / /
A simpler method of using positive feedback to reduce the
back-termination is shown in Figure 8. In this case, the
drivers are driven differentially and provide complemen-
tary outputs. Grounding the inputs, we see there is invert-
ing gain of –R
F
/R
P
from –V
o
to V
a
V
a
= V
o
(R
F
/R
P
)
Figure 8. Back-Termination Using Differential Positive Feedback
–
+
R
BT
R
F
R
G
R
P
R
P
R
G
R
L
R
L
–V
i
V
a
–V
a
V
i
–V
o
V
o
–
+
R
BT
1969 F08
R
F
R
L
n
=
V
o
V
i
n =
1 –2
FOR R
BT
=
R
F
R
P
R
F
R
P
+
R
F
R
G
1 +
1 –
R
F
R
P
1
()
15
LT1969
APPLICATIO S I FOR ATIO
WUUU
and assuming R
P
>> R
L
, we require
V
a
= V
o
(1 – 1/n)
solving
R
F
/R
P
= 1 – 1/n
So to reduce the back-termination by a factor of 3 choose
R
F
/R
P
= 2/3. Note that the overall gain is increased to:
V
o
/V
i
= (1 + R
F
/R
G
+ R
F
/R
P
)/[2(1 – R
F
/R
P
)]
ADSL Driver Requirements
The LT1969 is an ideal choice for ADSL upstream (CPE)
modems. The key advantages are: ±200mA output drive
with only 1.7V worst-case total supply voltage headroom,
high bandwidth, which helps achieve low distortion, low
quiescent supply current of 7mA per amplifier and a
space-saving, thermally enhanced MS10 package.
An ADSL remote terminal driver must deliver an average
power of 13dBm (20mW) into a 100Ω line. This corre-
sponds to 1.41V
RMS
into the line. The DMT-ADSL peak-to-
average ratio of 5.33 implies voltage peaks of 7.53V into
the line. Using a differential drive configuration and trans-
former coupling with standard back-termination, a trans-
former ratio of 1:2 is well suited. This is shown on the front
page of this data sheet along with the distortion perfor-
mance vs line voltage at 200kHz, which is beyond ADSL
requirements. Note that the distortion is better than
–73dBc for all swings up to 16V
P-P
into the line. The gain
of this circuit from the differential inputs to the line voltage
is 10. Lower gains are easy to implement using the
compensation techniques of Figure 5. Table 2 shows the
drive requirements for this standard circuit.
The above design is an excellent choice for desktop
applications and draws typically 550mW of power. For
portable applications, power savings can be achieved by
reducing the back-termination resistor using positive feed-
back as shown in Figure 9. The overall gain of this circuit
Figure 9. Power Saving ADSL Modem Driver
–
+
8.45Ω
1k
523Ω
1.21k
1.21k
523Ω
1µF
–V
i
V
i
A
V
= 10
–
+
8.45Ω
1969 F09
1k
1:1
100Ω
Table 2. ADSL Upstream Driver Designs
STANDARD LOW POWER
Line Impedance 100Ω 100Ω
Line Power 13dBm 13dBm
Peak-to-Average Ratio 5.33 5.33
Transformer Turns Ratio 2 1
Reflected Impedance 25Ω 100Ω
Back-Termination Resistors 12.5Ω 8.35Ω
Transformer Insertion Loss 1dB 0.5dB
Average Amplifier Swing 0.79V
RMS
0.87V
RMS
Average Amplifier Current 31.7mA
RMS
15mA
RMS
Peak Amplifier Swing 4.21V Peak 4.65V Peak
Peak Amplifier Current 169mA Peak 80mA Peak
Total Average Power Consumption 550mW 350mW
Supply Voltage Single 12V Single 12V
P1-P3P4-P6P7-P9P10-P12P13-P15P16-P18P19-P20

LT1969CMS#PBF

Mfr. #:
Buy LT1969CMS#PBF
Manufacturer:
Analog Devices Inc.
Description:
High Speed Operational Amplifiers 700MHz Dual 200mA OA with programmable I supply in MS10
Lifecycle:
New from this manufacturer.
Delivery:
DHL FedEx Ups TNT EMS
Payment:
T/T Paypal Visa MoneyGram Western Union

Products related to this Datasheet