Datasheet

LTC2487
22
2487fd
Figure 11. Equivalent Analog Input Circuit
IN
+
IN
–
10kΩ
INTERNAL
SWITCH
NETWORK
10kΩ
C
EQ
12μF
10kΩ
I
IN
–
REF
+
I
REF
+
I
IN
+
I
REF
–
2487 F11
SWITCHING FREQUENCY
f
SW
= 123kHz INTERNAL OSCILLATOR
f
SW
= 0.4 • f
EOSC
EXTERNAL OSCILLATOR
REF
–
10kΩ
100Ω
INPUT
MULTIPLEXER
100Ω
IIN
+
()
AVG
= IIN
–
()
AVG
=
V
IN(CM)
 V
REF(CM)
0.5•R
EQ
IREF
+
()
AVG

1.5V
REF
+ V
REF(CM)
–V
IN(CM)
()
0.5•R
EQ
–
V
IN
2
V
REF
•R
EQ
where:
V
REF
=REF
+
REF

V
REF(CM)
=
REF
+
–REF

2








V
IN
=IN
+
IN

,WHEREIN
+
ANDIN

ARE THE SELECTED INPUT CHANNELS
V
IN(CM)
=
IN
+
–IN

2








R
EQ
= 2.71M INTERNAL OSCILLATOR 60Hz MODE
R
EQ
= 2.98MINTERNAL OSCILLATOR 50Hz/60Hz MODE
R
EQ
= 0.833•10
12
()
/f
EOSC
EXTERNAL OSCILLATOR
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 simplifi ed 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.
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 fi lter unwanted noise
(antialiasing) 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.
APPLICATIONS INFORMATION