参数资料
型号: AD8155ACPZ
厂商: Analog Devices Inc
文件页数: 20/36页
文件大小: 0K
描述: IC MUX/DEMUX DUAL BUFFER 64LFCSP
标准包装: 1
系列: XStream™
功能: 多路复用器/多路分解器
电路: 2 x 2:1
电压电源: 单电源
电压 - 电源,单路/双路(±): 1.6 V ~ 3.6 V
电流 - 电源: 735mA
工作温度: -40°C ~ 85°C
安装类型: 表面贴装
封装/外壳: 64-VFQFN 裸露焊盘,CSP
供应商设备封装: 64-LFCSP-VQ(9x9)
包装: 托盘
AD8155
Rev. 0 | Page 27 of 36
OUTPUT COMPLIANCE
In low voltage applications, users must pay careful attention
to both the differential and common-mode signal levels. The
choice of output voltage swing, preemphasis setting, supply
voltages (VCC and VTTO), and output coupling (ac or dc) affect
peak and settled single-ended voltage swings and the common-
mode shift measured across the output termination resistors.
These choices also affect output current and, consequently,
power consumption. For certain combinations of supply voltage
and output coupling, output voltage swing and preemphasis
settings may violate the single-ended absolute output low
voltage, as specified in Table 1. Under these conditions, the
performance is degraded; therefore, these settings are not
recommended. Table 19 includes annotations that identify these
settings.
Table 19 shows the change in output common mode (ΔVOCM =
VCC VOCM) with output level (VSW) and preemphasis setting.
Table 19 also shows the minimum and maximum peak single-
ended output levels (VL-PE and VH-PE, respectively). The single-
ended output levels are calculated for VTTO supplies of 3.3 V and
1.8 V for both ac- and dc-coupled outputs to illustrate the
practical challenges of reducing the supply voltage.
TX_HEADROOM
For output levels greater than 400 mV diff (800 mV p-p diff),
setting the TX_HEADROOM bit to 1 allows the transmitter an
extra 200 mV of output compliance range. When the TX_
HEADROOM bit is enabled, a core supply voltage, VCC ≥ 2.5 V,
is required. Enabling TX_HEADROOM increases the core
supply current. TX_HEADROOM can be enabled on a per-port
basis through Bits[6:4] in Register 0x05. A value of 0 disables the
headroom-generating circuitry; a value of 1 enables it.
Example 1: 1.8 V, PE Disabled
Consider a typical application using pin control mode. In this
case, the default output level of 400 mV diff (800 mV p-p diff)
is selected, and the user can choose preemphasis settings of
0 dB or 6 dB. Table 19 shows that with preemphasis disabled,
a dc-coupled transmitter causes a 200 mV common-mode shift
across the termination resistors, whereas an ac-coupled transmitter
causes twice the common-mode shift. Notice that with VCC and
VTTO powered from a 1.8 V supply, the single-ended output voltage
swings between 1.8 V and 1.4 V when dc-coupled and between
1.6 V and 1.2 V when ac-coupled. In both cases, these levels are
greater than the minimum VL limit of 725 mV, and VCC satisfies
the minimum VCC limit of 1.8 V with the TX_HEADROOM bit
set to 0. Note that setting TX_HEADROOM = 1 violates the
minimum VCC limit of 2.5 V.
Example 2: 1.8 V, PE = 6 dB
With a PE setting of 6.02 dB, the ac-coupled transmitter has
single-ended swings from 1.4 V to 0.6 V, whereas the dc-
coupled transmitter outputs swing between 1.8 V and 1 V. The
peak minimum single-ended swing (VL-PE) of the ac-coupled
transmitter, in this case, exceeds the minimum VL limit of
725 mV by 125 mV. While theoretically in violation of the
specification, in practice, this setting is viable, especially at high
data rates. The transmitter theoretical peak voltage is rarely
achieved in practice because the high frequency characteristic
of the preemphasis is attenuated at the output pins by the low-
pass nature of the PC board environment and the channel. For
6.5 Gbps PE (SEL4G = 0), a 30% reduction of overshoot as
measured at the PC board is possible. For an output level of
400 mV diff and a PE setting of 6 dB, the user can calculate a
maximum overshoot of 400 mV diff but can measure only a
270 mV overshoot. With the preemphasis configured for
4.25 Gbps operation (SEL4G = 1), the measured overshoot
more closely matches the theoretical maximum. In this case, the
peak minimum voltage limit should be more closely observed.
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