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參數(shù)資料
| 型號： | AD1801 |
| 廠商： | Analog Devices, Inc. |
| 英文描述： | Single-Chip Fax/Data/ Voice Modem(單片的傳真/數(shù)據(jù)/語音調(diào)制解調(diào)器) |
| 中文描述： | 單芯片傳真/數(shù)據(jù)/語音調(diào)制解調(diào)器（單片的傳真/數(shù)據(jù)/語音調(diào)制解調(diào)器） |
| 文件頁數(shù)： | 50/52頁 |
| 文件大?。?/td> | 348K |
| 代理商： | AD1801 |
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AD1801
–50–
REV. 0
LOUT
HEADPHONE
OUTPUT
47k
V
68pF
NPO
20k
V
20k
V
220
m
F
1/2 OP279
2
125
CMOUT
Figure 29. Headphone Output Circuitry
Note: Due to the nature of the SRAM process used to fabricate
the AD1801, the digital and analog supplies for the device
MUST be derived from the same source.
24C04
A0
2
3
4
7
8
1
A1
A2
VSS
VCC
TEST
SCL
SDA
22k
V
43
42
SCK
SDATA
AD1801
5
6
Figure 30. Serial EEPROM (Two Wire) Circuit
DESIGN GUIDELINES
The analog and digital power pins on the AD1801 have to be
powered by the same supply to make sure that the analog power
pins are at the same dc potential as the digital power pins. Other-
wise substrate currents inside the AD1801 could exceed safe
limits and the device could be permanently damaged. In view of
this, it is recommended that the AD1801 be located over one
and the same power plane and each AD1801 power pin (DV
DD
and AV
DD
) be connected to that power plane. Other digital
devices on the PCB (at least ones driving AD1801 digital in-
puts) would typically be connected to this same power plane. It
is important that a low impedance, well bypassed power source
drive this power plane to reduce the chance of any DSP code
dependent noise appearing on the AD1801 analog signals. The
ISA/PCMCIA bus +5 V supply should suffice for this purpose.
In addition to bypassing each AD1801 supply pin, it is recom-
mended that an array of 0.001
μ
F, 1
μ
F ceramic and 10
μ
F
tantalum capacitors be connected between the power plane and
ground where power enters the PCB.
To minimize the digital ground currents flowing through the
analog area, it is recommended that two ground planes be used
for the AD1801, one for the digital half of the AD1801 plus
other digital circuitry (digital ground plane), and the other for
the analog half of the AD1801 plus any other analog circuitry
(analog ground plane). This is shown in Figure 31. The analog
and digital ground planes should only be tied together (mecca),
at the point where the supply enters the PCB. Instead of con-
necting the two ground planes directly (with a trace) on proto-
type PCBs, it is recommended that this connection be made
with a wire. This provides the flexibility to connect the two
ground planes through a ferrite bead (which increases the isola-
tion between the analog and digital circuitry), which may be
desirable in some systems.
Care should be taken to provide as clean supplies as possible
to the AD1801. At a minimum, an array of 10
μ
F, 0.1
μ
F and
0.01
μ
F capacitors should be used to filter the supply used to
power the AD1801 where it enters the PCB. A combination of
ferrite beads and capacitors can also be used to filter the supply,
but care should be observed when doing this, since a particular
combination of values, along with the power line supplying the
PCB, may form a high Q circuit that could produce undesirable
ringing of the supplies. As a general rule, regulating the +12 V
available on the ISA bus may provide the cleanest supplies for
the analog
circuitry. If a regulator is used, it should be located
close to where the unregulated voltage enters the PCB, at mecca
ground.
In addition to having the amplifiers that perform signal condi-
tioning on the DAA input also perform antialias filtering, it is
recommended that the user build their first system with addi-
tional RC antialias circuitry between the DAA amplifiers and
the DAA ADC input as shown in Figures 25 and 26. In addition
to providing residual antialias filtering, the resistors isolate the
DAA amplifiers from the switched capacitor DAA ADC inputs.
These components may be removed if performance testing
yields satisfactory results with the components not installed.
The audio inputs (MIC and LIN) should not need an antialias
filter in addition to what is provided by the amplifiers that drive
those inputs. Installing such RC filters on the audio inputs,
however, will only help improve the performance of the audio
channels, especially in a noisy environment.
To ensure the best performance, the AD1801 bypass capacitors
should be located as close to the AD1801 as possible. The fol-
lowing capacitors should be located as close to the AD1801 as
possible. Note that the positive side (preferably also the ground
side) of the supply bypass capacitors should be connected di-
rectly to the pin they are bypassing (i.e., not connected through
the power plane). This list is in the order of importance with the
capacitors that should be located closest listed first:
1. 0.1
μ
F ceramic capacitors bypassing V
REF
and CMOUT.
2. 0.1
μ
F ceramic capacitors on the voltage supply pins and the
22 pF capacitors on the crystal I/O pins.
3. Any antialias filter capacitors and ac coupling capacitors that
may be needed on the ADC inputs and the 1 nF ceramic
capacitor on the MSF pin.
4. 10
μ
F tantalum capacitors bypassing V
REF
and CMOUT and
the 1
μ
F tantalum capacitor connected to the FILT Pin.
5. Input and output ac coupling capacitors not mentioned
above do not need to be located close to the AD1801.
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