LTC2487 Linear Technology, LTC2487 Datasheet - Page 22

• Manufacturer Part Number: LTC2487
• Description: 16-Bit 2-/4-Channel ADC
• Manufacturer: Linear Technology

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LTC2487
applicaTions inForMaTion
Driving the Input and Reference
The input and reference pins of the LTC2487 are connected
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 11.
When using the LTC2487’s internal oscillator, the input
capacitor array is switched at 123kHz. The effect of the
charge transfer depends on the circuitry driving the
input/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.

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 11. Equivalent Analog Input Circuit
10kΩ
10kΩ
10kΩ
10kΩ
INTERNAL
NETWORK
SWITCH
2487 F11
Typically, the reference inputs are driven from a low
impedance source. In this case, complete settling occurs
even with large external bypass capacitors. The inputs
(CH0-CH3, COM), on the other hand, are typically driven
from larger source resistances. Source resistances up
to 10k may interface directly to the LTC2487 and settle
completely; however, the addition of external capacitors
at the input terminals in order to filter unwanted noise
(antialiasing) results in incomplete settling.
Automatic Differential Input Current Cancellation
In applications where the sensor output impedance is
low (up to 10kW with no external bypass capacitor or up
to 500W with 0.001µF bypass), complete settling of the
input occurs. In this case, no errors are introduced and
direct digitization is possible.
C
12µF
EQ
I IN
I I REF
R
R
R
where
V
V
V
V
( )
( )
REF
REF CM
IN
IN CM
EQ
EQ
EQ
(
=
+
(
=
=
=
=
IN
+
:
)
AVG
2 71
2.98M INTERNAL OSCILLATOR 50Hz/60
(
REF
0.833 10
+
=
.
) )
AVG
=
−




=
M INTERNAL OSCILLATOR




IN
+
IN WHERE IN AN
≈
REF
W
I IN
−
W
+
•
−
( )
1 5
REF
,
– –
2
.
+
–
IN
12
V
–
2
REF
−
−
AVG
)
REF




/f
EOSC
+
=
−
(
0 5
V
V
+




. •
IN CM
REF CM
EXTERNAL OSCIL
(
R
. 0 5
(
D D IN ARE THE SELECTED INPUT CHANNELS
E E Q
)
−
)
•
–
V
R
−
REF CM
EQ
V
IN CM
(
(
6
0 0 Hz MODE
)
)
)
L L ATOR
–
V
H H z MODE
REF
V
IN
•
2
R
EQ
2487fa