LTC2488 LINER [Linear Technology], LTC2488 Datasheet - Page 21

• Manufacturer Part Number: LTC2488
• Description: 16-Bit 2-/4-Channel ?? ADC with Easy Drive Input Current Cancellation
• Manufacturer: LINER [Linear Technology]

APPLICATIONS INFORMATION
CH3, COM), on the other hand, are typically driven from
larger source resistances. Source resistances up to 10k
may interface directly to the LTC2488 and settle completely;
however, the addition of external capacitors at the input
terminals in order to fi lter unwanted noise (anti-aliasing)
results in incomplete settling.
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 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 10kΩ bridge driving a
0.1µF capacitor has a time constant an order of magnitude
greater than the required maximum.
The LTC2488 uses a proprietary switching algorithm that
forces the average differential input current to zero indepen-
dent of external settling errors. This allows direct digitization
of high impedance sensors without the need of buffers.
SWITCHING FREQUENCY
f
f
SW
SW
REF
REF
I
I
IN +
IN
I
I
REF
REF
IN
IN
= 123kHz INTERNAL OSCILLATOR
= 0.4 • f
–
+
–
+
–
+
–
EOSC
MULTIPLEXER
INPUT
100Ω
100Ω
EXTERNAL OSCILLATOR
Figure 10. LTC2488 Equivalent Analog Input Circuit
10k
10k
10k
10k
INTERNAL
NETWORK
SWITCH
2488 F10
The switching algorithm forces the average input current
on the positive input (I
current on the negative input (I
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 balanced bridge, both the differential and com-
mon mode input currents 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 V
common mode voltage of 2.5V and an input common mode
of 1.5V, the common mode input current is approximately
0.74µA. This common mode input current does not degrade
the accuracy if the source impedances tied to IN
IN
C
12pF
+
EQ
– I
IN
I IN
I I REF
where
R
R
–
V
V
V
V
( )
( )
REF(CM)
REF
REF CM
IN
IN CM
EQ
E E Q
) is zero. While the differential input current is
(
=
+
(
=
=
=
IN
+
)
AVG
:
2.98M INTERNAL OSCILLATOR
(
REF
0.833 10
) )
+
=
AVG
=
−
⎛
⎜
⎜
⎝
IN(CM)
=
⎛
⎜
⎜
⎝
IN
+
IN WHERE IN AN
≈
REF
I IN
).
−
+
Ω
•
( )
−
1 5
REF
,
– – IN
2
.
–
+
12
V
–
2
IN(CM)
REF
−
−
AVG
)
) and the common mode reference
REF
IN
⎞
⎟
⎟
⎠
/f
EOSC
+
=
+
−
(
) to be equal to the average input
0 5
V
V
+
⎞
⎟
⎟
⎠
. •
IN CM
REF CM
EXTERNAL OSCILLATOR
(
and V
R
. 0 5
(
D D IN ARE THE SELECTED INPUT CHANNELS
E E Q
)
−
)
•
R
–
−
V
REF CM
EQ
V
IN
IN CM
REF(CM)
(
(
–
). Over the complete
)
)
)
–
V
LTC2488
REF
. For a reference
V
IN
•
IN
2
R
EQ
+
+ I
IN
21
–
+
)/2 is
and
2488f