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參數資料
| 型號： | AD8148ACPZ-RL |
| 廠商： | Analog Devices, Inc. |
| 英文描述： | Triple Differential Driver for Wideband Video |
| 中文描述： | 三差分驅動器的寬帶視頻 |
| 文件頁數： | 20/24頁 |
| 文件大小： | 438K |
| 代理商： | AD8148ACPZ-RL |
AD8146/AD8147/AD8148
Under no circumstances should capacitance be intentionally
added to an output to introduce frequency domain peaking.
Figure 36 and Figure 37 illustrate how adding just 5 pF of
excessive load capacitance influences time and frequency
domain responses.
Rev. 0 | Page 20 of 24
20
2.0
–2.0
0
TIME (ns)
V
1.5
1.0
0.5
0
–0.5
–1.0
–1.5
2
4
6
8
10
12
14
16
18
V
S
= ±5V
R
L, dm
= 200
V
OUT, dm
= 2V p-p
C
L
= 5pF
C
L
= 0pF
0
Figure 36. Large Signal Transient Responses at Various Capacitive Loads
12
2
10
1000
FREQUENCY (MHz)
G
100
11
10
9
8
7
6
5
4
3
V
S
= ±5V
R
L, dm
= 200
V
OUT, dm
= 2V p-p
C
L
= 5pF
C
L
= 0pF
0
Figure 37. Large Signal Frequency Responses at Various Capacitive Loads
While high frequency peaking is desirable in some cable
equalization applications, it should be implemented using
methods that do not compromise the stability of the driver and
that do not depend on amplifier parasitic elements. The parasitic
elements are affected by process variations and cannot be
depended upon for circuit designs. The amplifier may break
into oscillation when excess load capacitance is intentionally
added. For more information on this topic, see the Adding Pre-
Emphasis to the AD8148 section for a description on how to
introduce a controlled amount of pre-emphasis for 30 meters of
UTP using the AD8148.
ADDING PRE-EMPHASIS TO THE AD8148
UTP cables exhibit loss characteristics that are low pass in
nature and are exponential functions of the square root of the
frequency. Over wideband video bandwidths, the losses are
predominantly due to the skin effect, which causes the resistance of
the cable to increase with frequency. Even though the loss
characteristics are nonlinear, suitable linear networks can be
designed to approximately compensate for the losses.
Placing the compensation network at the transmitting end of
the cable is referred to as pre-emphasis, because the higher
frequencies are emphasized, or boosted, before they are sent, to
compensate for the low-pass response of the cable. Because the
higher frequencies experience more loss than the lower frequencies
as they pass through the cable, the high and low frequencies
arrive at approximately the same level and at the end of the cable
when a properly designed pre-emphasis network is used at the
transmitter. The ideal cascaded frequency response of the pre-
emphasis network and the cable is therefore nominally flat.
Because the AD8148 has an internally set, closed-loop gain of 4
(12 dB), it is possible to reduce the gain at low frequencies using
external frequency selective components, then use these
components to provide increasing gain with increasing
frequency, back to a value close to 12 dB. These components, along
with the AD8148, form the pre-emphasis network. When properly
designed, the combined frequency response of the pre-emphasis
network and cable is approximately flat with a gain of 2 (6 dB).
Figure 38 illustrates how to construct a pre-emphasis network
using the AD8148 that compensates for 30 meters of UTP cable.
The network in the lower leg is required to match the transfer
function of the two feedback loops.
At dc, the capacitors are open circuits, and the network has a
gain of approximately 6.5 dB. (The additional 0.5 dB is added to
compensate for the cable flat loss that occurs at frequencies
below where the skin effect begins to take effect.) Moving up in
frequency, the 30 pF capacitor begins to take effect and introduces
a zero into the frequency response, causing the gain to increase
with frequency. Continuing to move up in frequency, the 30 pF
capacitor becomes an effective short, and the 487 Ω resistor
goes in parallel with the 442 Ω resistor, forming a pole in the
response. Continuing to move up in frequency, the 15 pF
capacitor takes effect, introducing another zero, and causes
the gain to further increase with frequency until it becomes
an effective short, and the gain starts to flatten out until the
amplifier response begins to roll off. The gain does not reach
12 dB before the amplifier begins to roll off because the 12 dB
value is a high frequency asymptote. The pole and zero locations
cited in the previous discussion are qualitative, but the
discussion describes the basic principles involved with the
operation of the pre-emphasis network.
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