Datasheet

LTC2492
27
2492fd
APPLICATIONS INFORMATION
Driving the Input and Reference
The input and reference pins of the LTC2492 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 12.
When using the LTC2492’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
to CH3, COM), on the other hand, are typically driven from
larger source resistances. Source resistances up to 10k
may interface directly to the LTC2492 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 LTC2492 uses a proprietary switching algorithm that forces
the average differential input current to zero independent of
external settling errors. This allows direct digitization of high
impedance sensors without the need of buffers.
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
Figure 12. LTC2492 Equivalent Analog Input Circuit
IN
+
IN
–
10k
INTERNAL
SWITCH
NETWORK
10k
C
EQ
12pF
10k
I
IN
–
REF
+
I
REF
+
I
IN
+
I
REF
–
2492 F12
SWITCHING FREQUENCY
f
SW
= 123kHz INTERNAL OSCILLATOR
f
SW
= 0.4 • f
EOSC
EXTERNAL OSCILLATOR
REF
–
10k
100Ω
INPUT
MULTIPLEXER
100Ω
IIN IIN
VV
R
AVG AVG
IN CM REF CM
EQ
+
()
=
()
=
−
•
–
() ()
.05
IIREF
VV V
R
AVG
REF REF CM IN CM
+
()
≈
+
()
15
05
.–
.•
() ()
EEQ
IN
REF EQ
REF
REF CM
V
VR
where
V REF REF
V
–
•
:
(
2
=−
+−
))
–
,
=
⎛
⎝
⎜
⎜
⎞
⎠
⎟
⎟
=−
+−
+− +
REF REF
V IN IN WHERE IN AN
IN
2
DD IN ARE THE SELECTEDINPUT CHANNELS
V
IN
IN CM
−
+
=
()
––
.
IN
R M INTERNAL OSCILLATOR
EQ
−
⎛
⎝
⎜
⎜
⎞
⎠
⎟
⎟
=
2
271 6Ω 00Hz MODE
R 2.98 M INTERNAL OSCILLATOR 50Hz/60
EQ
=Ω HHz MODE
R0.833 10 /f EXTERNAL OSCIL
EQ
12
EOSC
=•
()
LLATOR