ltc2484 Linear Technology Corporation, ltc2484 Datasheet - Page 27

• Manufacturer Part Number: ltc2484
• Description: 24-bit Delta Sigma Adc With Easy Drive Input Current Cancellation
• Manufacturer: Linear Technology Corporation

APPLICATIO S I FOR ATIO
Automatic Differential Input Current Cancellation
In applications where the sensor output impedance is low
(up to 10kΩ with no external bypass capacitor or up to
500Ω with 0.001µF bypass), complete settling of the input
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 10kΩ bridge driving a 0.1µF
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
power. The LTC2484 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 for 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
current on the negative input (I
SWITCHING FREQUENCY
f
f
SW
SW
V
V
GND
I
V
I
REF
REF
I
I
REF
IN
IN
IN
IN
= 123kHz INTERNAL OSCILLATOR
= 0.4 • f
+
+
–
–
+
+
–
V
V
EOSC
CC
CC
V
V
EXTERNAL OSCILLATOR
I
I
I
I
LEAK
LEAK
LEAK
LEAK
CC
CC
I
I
I
I
LEAK
LEAK
LEAK
LEAK
CC
U
), added system cost and increased
R
R
R
R
SW
SW
SW
SW
IN
10k
10k
10k
10k
(TYP)
(TYP)
(TYP)
(TYP)
+
) to be equal to the average input
U
2484 F11
IN
–
W
). Over the complete
Figure 11. LTC2484 Equivalent Analog Input Circuit
C
12pF
(TYP)
EQ
U
conversion cycle, the average differential input current
(I
zero, the common mode input current (I
proportional to the difference between the common mode
input voltage (V
voltage (V
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
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
common mode of 2.5V and an input common mode of
1.5V, the common mode input current is approximately
0.74µA (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
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 1kΩ source resistances
leads to a 15ppm shift (74µV) in offset voltage.
IN
+
– I
IN
–
REFCM
) is zero. While the differential input current is
).
INCM
) and the common mode reference
INCM
and V
REFCM
. For a reference
LTC2484
IN
+
+
and IN
+ I
+
IN
and IN
27
–
)/2 is
–
2484fa
are
–