參數(shù)資料
型號: DC939A
廠商: Linear Technology
文件頁數(shù): 27/42頁
文件大小: 0K
描述: BOARD DELTA SIGMA ADC LTC2484
軟件下載: QuikEval System
設(shè)計資源: DC939A Design File
DC939A Schematic
標準包裝: 1
系列: Easy Drive™, QuikEval™
ADC 的數(shù)量: 1
位數(shù): 24
采樣率(每秒): 6.8
數(shù)據(jù)接口: MICROWIRE?,串行,SPI?
工作溫度: 0°C ~ 70°C
已用 IC / 零件: LTC2484
已供物品:
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LTC2484
33
2484fd
APPLICATIONS INFORMATION
at the 3dB frequency. When the internal oscillator is used,
the shape of the LTC2484 input bandwidth is shown in
Figure 28. When an external oscillator of frequency fEOSC
is used, the shape of the LTC2484 input bandwidth can
be derived from Figure 28, 60Hz mode curve in which the
horizontal axis is scaled by fEOSC/307200.
The conversion noise (600nVRMS typical for VREF = 5V)
can be modeled by a white noise source connected to
a noise free converter. The noise spectral density is
47nV√Hz for an infinite bandwidth source and 64nV√Hz
for a single 0.5MHz pole source. From these numbers,
it is clear that particular attention must be given to
the design of external amplification circuits. Such
circuits face the simultaneous requirements of very
low bandwidth (just a few Hz) in order to reduce the
output referred noise and relatively high bandwidth (at
least 500kHz) necessary to drive the input switched-
capacitor network. A possible solution is a high gain, low
bandwidth amplifier stage followed by a high bandwidth
unity-gain buffer.
When external ampliers are driving the LTC2484, the
ADC input referred system noise calculation can be
simplied by Figure 29. The noise of an amplier driving
the LTC2484 input pin can be modeled as a band limited
white noise source. Its bandwidth can be approximated
by the bandwidth of a single pole lowpass lter with a
corner frequency fi. The amplier noise spectral density
is ni. From Figure 29, using fi as the x-axis selector, we
can nd on the y-axis the noise equivalent bandwidth freqi
of the input driving amplier. This bandwidth includes
the band limiting effects of the ADC internal calibration
and ltering. The noise of the driving amplier referred
to the converter input and including all these effects can
be calculated as N = ni √freqi. The total system noise
(referred to the LTC2484 input) can now be obtained by
summing as square root of sum of squares the three ADC
input referred noise sources: the LTC2484 internal noise,
the noise of the IN+ driving amplier and the noise of the
INdriving amplier.
If the fO pin is driven by an external oscillator of frequency
fEOSC, Figure 29 can still be used for noise calculation if
the x-axis is scaled by fEOSC/307200. For large values of
the ratio fEOSC/307200, the Figure 29 plot accuracy begins
to decrease, but at the same time the LTC2484 noise oor
rises and the noise contribution of the driving ampliers
lose signicance.
DIFFERENTIAL INPUT SIGNAL FREQUENCY (Hz)
0
INPUT
SIGNAL
ATTENUATION
(dB)
–3
–2
–1
0
4
2484 F28
–4
–5
–6
1
2
3
5
50Hz MODE
60Hz MODE
50Hz AND
60Hz MODE
Figure 28. Input Signal Bandwidth Using the Internal Oscillator
INPUT NOISE SOURCE SINGLE POLE
EQUIVALENT BANDWIDTH (Hz)
1
INPUT
REFERRED
NOISE
EQUIVALENT
BANDWIDTH
(Hz)
10
0.1
1
10
100
1k
10k
100k
1M
2484 F29
0.1
100
50Hz MODE
60Hz MODE
Figure 29. Input Referred Noise Equivalent Bandwidth
of an Input Connected White Noise Source
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