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參數(shù)資料
| 型號(hào)： | AN504 |
| 廠商： | Vishay Intertechnology,Inc. |
| 英文描述： | Video Crosspoint Switch Simplifies Large Matrix Designs |
| 中文描述： | 視頻交叉點(diǎn)開關(guān)簡(jiǎn)化大型矩陣設(shè)計(jì) |
| 文件頁(yè)數(shù)： | 3/12頁(yè) |
| 文件大?。?/td> | 161K |
| 代理商： | AN504 |
AN504
Vishay Siliconix
Document Number: 70610
05-Aug-99
www.vishay.com FaxBack 408-970-5600
6-3
Transmission Loss
Transmission loss is a function of the load connected to the
crosspoint output for a fixed channel on-resistance and source
impedance. Transmission loss is given by
Loss (dB)
20 log
10
R
load
R
channel
R
source
R
load
Where R
channel
= R
DS1
+ R
DS2
+ R
DS4
.
This equation is accurate at frequencies below 1 MHz, where
device capacitances (i.e., on-state input capacitance) can be
safely ignored.
Input Overvoltage
Overvoltage effects are the third important characteristic that
can be visualized from the models shown in Figures 3 and 4.
Unlike mechanical switches, solid-state analog switches have
electrical “end-stops” beyond which the switch ceases to
behave as a switch. These “end-stops” are defined by the
power supply rails. For an ”on” channel, a signal at the input
can swing to the negative rail before the shunt-switch body
diodes (Figure 3) begin to conduct. When these diodes
conduct, a large current from the negative rail can result. This
current flow should be limited to 20 mA dc as defined in the
absolute maximum ratings. During the period of negative
overvoltage, the crosspoint outputs will be corrupted whether
they are selected or not. A single diode in series with the
negative rail will prevent reverse current flow and output
corruption during negative overvoltage.
In the positive direction, if the input is allowed to continue rising
towards the positive supply rail, its amplitude should be limited
to the breakdown voltage of the body-to-source junction of the
switching FETs. This is specified in the data sheet maximum
ratings section, where a value of 14 V above the negative rail
is quoted. Where rails of +15 V and –5 V are used, the
body-source breakdown voltage is the maximum positive limit
of the signal at the source (input) or the drain (output) terminals.
In a power down situation, where the positive and negative
rails are at zero, the maximum positive input signal is also 14 V
above the negative rail. No output corruption will occur during
positive overvoltage.
Distortion
The change of on-resistance with signal swing, explained
above, will give rise to distortion unless specific precautions
are taken. A study of the equation for transmission loss reveals
that distortion caused by changing on-resistance is inversely
proportional to load resistance. Thus, distortion is given by
DIST = 20 x log ( R
on
/R
load
)
From the above, the decibel level below the fundamental may
be calculated. Figure 5 shows how the on-resistance varies
with signal swing. R
on
is 5 for a signal swing of 5 V. Thus,
for R
load
= 5 k , distortion is –60 dB. For low-frequency
operation of the DG884 (where isolation and crosstalk
performance may be critical in demanding applications), signal
swings are usually much greater than those found at video
frequencies and above. Thus, R
load
should be increased
accordingly. In all cases, some form of high-performance
buffer/amplifier will be required at the output to translate the
crosspoint load resistance to a lower “interface” value, i.e., 50
to 75 for high-frequency designs.
200
0
2
4
6
8
180
20
160
140
120
100
80
60
40
–2
10
r
DS(on)
vs. Drain Voltage
V+ = 15 V
V– = –3 V
V
L
= 5 V
I
S
= –10 mA
V
D
– (V)
FIGURE 5.
Change of On-Resistance with Signal Amplitude
r
)
125 C
25 C
–55 C
Decoupling
A high-frequency signal path exists between the analog
channel and the power rails via the gate-to-source
capacitances of the switching FETs, as shown in Figures 3 and
4. This is because the drive logic applies either rail voltage to
the gate via a low impedance path. Good decoupling shunts
this signal leakage harmlessly to ground.
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