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參數(shù)資料
| 型號： | AD776 |
| 廠商： | Analog Devices, Inc. |
| 英文描述： | 16-Bit 100 kSPS Oversampling ADC(16位100kSPS過采樣A/D轉(zhuǎn)換器) |
| 中文描述： | 16位100 kSPS的采樣ADC的（16位100ksps的過采樣的A / D轉(zhuǎn)換器） |
| 文件頁數(shù)： | 9/12頁 |
| 文件大?。?/td> | 257K |
| 代理商： | AD776 |
AD776
REV. A
–9–
20
19
300
100
μ
F
REFOUT
REFIN
+5V
AD776
AD680
0.1
μ
F
2.0V
75
NO CONNECT
2.5V
Figure 11. External Voltage Reference Circuit
MULT IPLE X ING
T he AD776 can also be used with an input multiplexer when
the comb filter output is selected by setting FSEL = 1. If f
CLK
= 12.8 MHz, the minimum multiplex intervals are (including
the time to shift the data out from the serial interface):
15
μ
s
(if the FSI and mux are perfectly synchronized)
17.5
μ
s
(if the FSI and mux are not synchronized).
CLOCK GE NE RAT ION
With sigma-delta converters, it is critical that clock jitter be
minimized in order to achieve optimal performance. Figure 12
illustrates a simple circuit used to derive a clock source for the
AD776. An alternative would be to use an oscillator such as the
CK 1100 series from Cardinal Components (Montclair, NJ) or
the F1100 from Fox Electronics. Compared with performance
obtained with a typical crystal, use of an oscillator improves
SNR by approximately 4 dB.
TO AD776
PIN 4 (CLKIN)
74HCU04
74HCU04
68pF
330
6.144MHz
470k
68pF
Figure 12. Basic Clock Circuit
BOARD LAY OUT
Designing with high resolution data converters requires careful
attention to board layout. T race impedance is a significant is-
sue. A 1.22 mA current through a 0.5
trace will develop a
voltage drop of 0.6 mV, which is 20 LSBs at the 16 bit level for
a 2 V full-scale span. In addition to ground drops, inductive
and capacitive coupling need to be considered, especially when
high accuracy analog signals share the same board with digital
signals.
Analog and digital signals should not share a common return
path. Each signal should have an appropriate analog or digital
return routed close to it. Using this approach, signal loops
enclose a small area, minimizing the inductive coupling of
noise. Wide PC tracks, large gauge wire, and ground planes are
highly recommended to provide low impedance signal paths.
Separate analog and digital ground planes are also desirable,
with a single interconnection point at (or under) the part to
minimize ground loops. T his is preferred to interconnecting the
grounds at the supplies.
Analog signals should be routed as far as possible from digital
signals and should cross them, if at all, only at right angles. T he
AD776 may be treated as an analog component, with both
AGND and DGND connected to a single analog ground plane.
T his helps to isolate the AD776 from large digital ground cur-
rents. For these reasons, the use of wire wrap circuit construc-
tion will not provide adequate performance; careful printed
circuit board construction is preferred.
POWE R SUPPLIE S AND DE COUPLING
With high performance linear circuits, changes in the power
supplies can produce undesired changes in the performance of
the circuit. Optimally, well regulated power supplies with less
than 1% ripple should be selected. T he ac output impedance of
a power supply is a complex function of frequency, but in gen-
eral will increase with frequency. High frequency switching such
as that encountered with digital circuitry requires fast transient
currents which most power supplies cannot adequately provide.
T his results in voltage spikes on the supplies. T o compensate
for the finite ac output impedance of the supplies, it is necessary
to store “reserves” of charge in bypass capacitors. T hese capaci-
tors can effectively lower the ac impedance presented to the
AD776 power inputs which in turn will significantly reduce the
magnitude of the voltage spikes. Decoupling capacitors, typi-
cally 0.1
μ
F, should be placed as close as possible to each power
supply pin of the AD776. It is essential that these capacitors be
placed physically close to the AD776 to minimize the induc-
tance of the PCB trace between the capacitor and the supply
pin.
Additionally, it is beneficial to have large capacitors (>47
μ
F)
located at the point where the power connects to the PCB with
10
μ
F capacitors located in the vicinity of the ADC to further
reduce low frequency ripple.
T he AD776 may be operated from a single +5 V supply. How-
ever, performance is optimized by using separate analog (AV
DD
)
and digital (DV
DD
) supplies. Separate supplies enable isolation
of digital noise from the analog circuitry. When separate sup-
plies are used, AV
DD
should be decoupled to analog ground
(AGND) and DV
DD
should be decoupled to digital ground
(DGND) with decoupling capacitors.
When a single +5 V supply is used, the circuit shown in Figure
13 provides adequate decoupling.
+5V
AD776
0.1
μ
F
10
μ
F
AV
DD
DV
DD
0.01
μ
F
FERRITE
BEAD
10
0.1
μ
F
Figure 13. Single Supply Decoupling
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