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參數(shù)資料
| 型號(hào)： | LTC2494IUHF |
| 廠商： | LINEAR TECHNOLOGY CORP |
| 元件分類： | ADC |
| 英文描述： | 16-CH 16-BIT DELTA-SIGMA ADC, SERIAL ACCESS, PQCC38 |
| 封裝： | 5 X 7 MM, PLASTIC, MO-220WKHD, QFN-38 |
| 文件頁數(shù)： | 21/36頁 |
| 文件大?。?/td> | 675K |
| 代理商： | LTC2494IUHF |
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LTC2494
2494fa
applicaTions inFormaTion
For many applications, the sensor output impedance
combined with external input bypass capacitors produces
RC time constants much greater than the 580ns requied
for 1ppm accuracy. For example, a 10k bridge driving a
0.1F capacitor has a time constant an order of magnitude
greater than the required maximum.
The LTC2494 uses a propriatary switching algorithm
that forces the average differential input current to zero
independent of external settling errors. This allows direct
digitization 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 in the negative input (IIN–). Over the complete
conversion cycle, the average input current (IIN+ – IIN–)
is zero. While the differential input current is zero, the
common mode input current (IIN+ + IIN–)/2 is proportional
to the difference between the common mode input volt-
age (VIN(CM)) and the common mode reference voltage
(VREF(CM)).
In applications where the input common mode voltage is
equal to the reference common mode voltage, as in the
case of a balanced bridge, both the differential and com-
mon mode input current are zero. The accuracy of the
converter is not compromised by settling errors.
In applications where the input common mode voltage is
constant but different from the reference common mode
voltage, the differential input current remains zero while
the common mode input current is proportional to the
difference between VIN(CM) and VREF(CM). For a reference
commonmodevoltageof2.5Vandaninputcommonmode
of 1.5V, the common mode input current is approximately
0.74A (in simultaneous 50Hz/60Hz rejection mode). This
commonmodeinputcurrentdoesnotdegradetheaccuracy
if the source impedances tied to IN+ and IN– are matched.
Mismatches in source impedance lead to a fixed offset
error but do not effect the linearity or full scale reading.
A 1% mismatch in a 1k source resistance leads to a 74V
shift in offset voltage.
In applications where the common mode input voltage
varies as a function of the input signal level (single ended
type sensors), the common mode input current varies
proportionally with input voltage. For the case of balanced
inputimpedances,thecommonmodeinputcurrenteffects
are rejected by the large CMRR of the LTC2494, leading
to little degradation in accuracy. Mismatches in source
Figure 12. LTC2494 Equivalent Analog Input Circuit
IN+
IN–
10k
INTERNAL
SWITCH
NETWORK
10k
CEQ
12pF
10k
IIN–
REF+
IREF+
IIN+
IREF–
2494 F12
SWITCHING FREQUENCY
fSW = 123kHz INTERNAL OSCILLATOR
fSW = 0.4 fEOSC EXTERNAL OSCILLATOR
REF–
10k
100
INPUT
MULTIPLEXER
EXTERNAL
CONNECTION
100
MUXOUTP
ADCINP
EXTERNAL
CONNECTION
MUXOUTN
ADCINN
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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