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參數(shù)資料
| 型號： | LTC2492IDE#TR |
| 廠商： | LINEAR TECHNOLOGY CORP |
| 元件分類： | ADC |
| 英文描述： | 4-CH 24-BIT DELTA-SIGMA ADC, SERIAL ACCESS, PDSO14 |
| 封裝： | 4 X 3 MM, PLASTIC, MO-229WGED-3, QFN-14 |
| 文件頁數(shù)： | 20/36頁 |
| 文件大小： | 652K |
| 代理商： | LTC2492IDE#TR |
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LTC2492
2492fa
applicaTions inFormaTion
Driving the Input and Reference
TheinputandreferencepinsoftheLTC2492areconnected
directly to a switched capacitor network. Depending on
the relationship between the differential input voltage and
the differential reference voltage, these capacitors are
switched between these four pins. Each time a capacitor
is switched between two of these pins, a small amount
of charge is transferred. A simplified equivalent circuit is
shown in Figure 12.
When using the LTC2492’s internal oscillator, the input
capacitor array is switched at 123kHz. The effect of the
charge transfer depends on the circuitry driving the in-
put/reference pins. If the total external RC time constant
is less than 580ns the errors introduced by the sampling
process are negligible since complete settling occurs.
Typically,thereferenceinputsaredrivenfromalowimped-
ance source. In this case, complete settling occurs even
with large external bypass capacitors. The inputs (CH0 to
CH3, COM), on the other hand, are typically driven from
larger source resistances. Source resistances up to 10k
mayinterfacedirectlytotheLTC2492andsettlecompletely;
however, the addition of external capacitors at the input
terminals in order to filter unwanted noise (anti-aliasing)
results in incomplete settling.
Automatic Differential Input Current Cancellation
In applications where the sensor output impedance is
low (up to 10k
Ω with no external bypass capacitor or up
to 500
Ω with 0.001F bypass), complete settling of the
input occurs. In this case, no errors are introduced and
direct digitization is possible.
For many applications, the sensor output impedance
combined with external input bypass capacitors produces
RC time constants much greater than the 580ns required
for 1ppm accuracy. For example, a 10k
Ω bridge driving a
0.1F capacitor has a time constant an order of magnitude
greater than the required maximum.
The LTC2492 uses a proprietary switching algorithm that
forcestheaveragedifferentialinputcurrenttozeroindepen-
dentofexternalsettlingerrors.Thisallowsdirectdigitization
of high impedance sensors without the need of buffers.
The switching algorithm forces the average input current
on the positive input (IIN+) to be equal to the average input
current on the negative input (IIN–). Over the complete
Figure 12. LTC2492 Equivalent Analog Input Circuit
IN+
IN–
10k
INTERNAL
SWITCH
NETWORK
10k
CEQ
12pF
10k
IIN–
REF+
IREF+
IIN+
IREF–
2492 F12
SWITCHING FREQUENCY
fSW = 123kHz INTERNAL OSCILLATOR
fSW = 0.4 fEOSC EXTERNAL OSCILLATOR
REF–
10k
100
INPUT
MULTIPLEXER
100
I IN
V
R
AVG
IN CM
REF CM
EQ
+
( ) = ( ) =
–
(
)
(
)
.
0 5
II REF
V
R
AVG
REF
REF CM
IN CM
+
( ) ≈
+
(
)
1 5
0 5
.
–
.
(
)
(
)
EEQ
IN
REF
EQ
REF
REF CM
V
R
where
V
REF
V
–
:
(
2
=
+
))
–
,
=
=
+
+
+
REF
V
IN
IN WHERE IN AN
IN
2
D
DIN ARE THE SELECTEDINPUT CHANNELS
V
IN
IN CM
+
=
(
)
––
.
IN
R
M INTERNAL OSCILLATOR
EQ
=
2
2 71
6
Ω
00Hz MODE
R
2.98M INTERNAL OSCILLATOR 50Hz/60
EQ =
Ω
H
Hz MODE
R
0.833 10
/f
EXTERNAL OSCIL
EQ
12
EOSC
=
(
)
LLATOR
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