參數(shù)資料
型號(hào): LTC2480CDD
廠商: LINEAR TECHNOLOGY CORP
元件分類: ADC
英文描述: 16-Bit ADC with Easy Drive
中文描述: 1-CH 16-BIT DELTA-SIGMA ADC, SERIAL ACCESS, PDSO10
封裝: 3 X 3 MM, PLASTIC, MO-229WEED-2, DFN-10
文件頁(yè)數(shù): 33/40頁(yè)
文件大?。?/td> 535K
代理商: LTC2480CDD
33
LTC2480
2480f
APPLICATIU
Due to the complex filtering and calibration algorithms
utilized, the converter input bandwidth is not modeled
very accurately by a first order filter with the pole located
at the 3dB frequency. When the internal oscillator is used,
the shape of the LTC2480 input bandwidth is shown in
Figure 28. When an external oscillator of frequency f
EOSC
is used, the shape of the LTC2480 input bandwidth can be
derived from Figure 28, 60Hz mode curve in which the
horizontal axis is scaled by f
EOSC
/307200.
The conversion noise (600nV
RMS
typical for V
REF
= 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 amplifiers are driving the LTC2480, the
ADC input referred system noise calculation can be sim-
plified by Figure 29. The noise of an amplifier driving the
LTC2480 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 filter with a corner
frequency f
i
. The amplifier noise spectral density is n
i
.
From Figure 29, using f
i
as the x-axis selector, we can find
on the y-axis the noise equivalent bandwidth freq
i
of the
input driving amplifier. This bandwidth includes the band
limiting effects of the ADC internal calibration and filter-
ing. The noise of the driving amplifier referred to the
converter input and including all these effects can be
calculated as N = n
i
freq
i
. The total system noise
(referred to the LTC2480 input) can now be obtained by
summing as square root of sum of squares the three ADC
input referred noise sources: the LTC2480 internal noise,
the noise of the IN
+
driving amplifier and the noise of the
IN
driving amplifier.
If the F
O
pin is driven by an external oscillator of frequency
f
EOSC
, Figure 29 can still be used for noise calculation if the
W
U
U
x-axis is scaled by f
EOSC
/307200. For large values of the
ratio f
EOSC
/307200, the Figure 29 plot accuracy begins to
decrease, but at the same time the LTC2480 noise floor
rises and the noise contribution of the driving amplifiers
lose significance.
Normal Mode Rejection and Antialiasing
One of the advantages delta-sigma ADCs offer over con-
ventional ADCs is on-chip digital filtering. Combined with
a large oversampling ratio, the LTC2480 significantly
simplifies antialiasing filter requirements. Additionally,
the input current cancellation feature of the LTC2480
allows external lowpass filtering without degrading the DC
performance of the device.
Figure 29. Input Referred Noise Equivalent Bandwidth
of an Input Connected White Noise Source
DIFFERENTIAL INPUT SIGNAL FREQUENCY (Hz)
0
I
–3
–2
–1
0
4
2480 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
I
E
10
0.1
1
10
100
1k
10k
100k
1M
2480 F29
0.1
100
50Hz MODE
60Hz MODE
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