參數(shù)資料
型號: LTC2480
廠商: Linear Technology Corporation
英文描述: 16-Bit ADC with Easy Drive
中文描述: 16位ADC的易道
文件頁數(shù): 27/40頁
文件大?。?/td> 535K
代理商: LTC2480
27
LTC2480
2480f
occurs. In this case, no errors are introduced and direct
digitization of the sensor is possible.
For many applications, the sensor output impedance com-
bined with external bypass capacitors produces RC time
constants much greater than the 580ns required for 1ppm
accuracy. For example, a 10k
bridge driving a 0.1
μ
F
bypass capacitor has a time constant an order of magni-
tude greater than the required maximum. Historically,
settling issues were solved using buffers. These buffers
led to increased noise, reduced DC performance (Offset/
Drift), limited input/output swing (cannot digitize signals
near ground or V
CC
), added system cost and increased
power. The LTC2480 uses a proprietary switching algo-
rithm that forces the average differential input current to
zero independent of external settling errors. This allows
accurate direct digitization of high impedance sensors
without the need of buffers. Additional errors resulting
from mismatched leakage currents must also be taken into
account.
The switching algorithm forces the average input current
on the positive input (I
IN+
) to be equal to the average input
current on the negative input (I
IN–
). Over the complete
conversion cycle, the average differential input current
(I
IN+
– I
IN–
) is zero. While the differential input current is
zero, the common mode input current (I
IN+
+ I
IN–
)/2 is
proportional to the difference between the common mode
input voltage (V
INCM
) and the common mode reference
voltage (V
REFCM
).
In applications where the input common mode voltage is
equal to the reference common mode voltage, as in the
case of a balance bridge type application, both the differ-
ential and common mode input current are zero. The
accuracy of the converter is unaffected by settling errors.
Mismatches in source impedances between IN
+
and IN
also do not affect the accuracy.
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 V
INCM
and V
REFCM
. For a reference
common mode of 2.5V and an input common mode of
1.5V, the common mode input current is approximately
0.74
μ
A (in simultaneous 50Hz/60Hz rejection mode). This
common mode input current has no effect on the accuracy
if the external source impedances tied to IN
+
and IN
are
matched. Mismatches in these source impedances lead to
a fixed offset error but do not affect the linearity or full-
scale reading. A 1% mismatch in 1k
source resistances
leads to a 15ppm shift (74
μ
V) in offset voltage.
APPLICATIU
W
U
U
Figure 11. LTC2480 Equivalent Analog Input Circuit
V
REF
+
V
IN
+
V
CC
R
SW
(TYP)
10k
I
LEAK
I
LEAK
V
CC
V
CC
I
LEAK
I
LEAK
R
SW
(TYP)
10k
C
12pF
(TYP)
R
SW
(TYP)
10k
I
LEAK
I
IN+
V
IN–
I
IN–
I
REF+
I
REF–
2480 F11
I
LEAK
V
CC
I
LEAK
I
LEAK
SWITCHING FREQUENCY
f
SW
= 123kHz INTERNAL OSCILLATOR
f
SW
= 0.4 f
EOSC
EXTERNAL OSCILLATOR
GND
R
SW
(TYP)
10k
I IN
I IN
V
V
R
IREF
V
V
V
R
V
V
R
V
R
D
V
V
0 5
V
R
V
V
R
where
AVG
)
AVG
IN CM
(
REF CM
(
EQ
AVG
REF
V
REF
2
INCM
REFCM
EQ
IN
REF
EQ
REF
T
EQ
REF
REF CM
(
.
IN CM
(
EQ
IN
REF
EQ
+
+
)
=
)
=
=
.
+
+
(
)
)
.
)
)
)
.
.
0 5
.
0 5
1 5
0 5
1 5
2
2
:
.
V
V
IN
IN
V
IN
IN
R
R
M
INTERNAL OSCILLATOR
INTERNAL OSCILLATOR 50Hz AND 60Hz MODE
HzMODE
60
REFCM
IN
INCM
EQ
=
=
=
+
2
=
=
=
(
)
+
+
+
2 71
2.98M
R
0.833 10
/ f
EXTERNAL OSCILLATOR
D IS THE DENSITY OF A DIGITAL TRANSITION AT THE MODULATOR OUTPUT
WHERE REF
IS INTERNALLY TIED TO GND
EQ
EQ
12
EOSC
相關(guān)PDF資料
PDF描述
LTC2480IDD 16-Bit ADC with Easy Drive
LTC2480CDD 16-Bit ADC with Easy Drive
LTC2486 16-Bit 2-/4-Channel ツヒ ADC with PGA and Easy Drive Input Current Cancellation
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