Datasheet

LTC2498
28
2498ff
applications inForMation
Driving the Input and Reference
The input and reference pins of the LTC2498 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 12.
When using the LTC2498’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
then 580ns the errors introduced by the sampling process
are negligible since complete settling occurs.
Typically, the reference inputs are driven from a low imped-
ance source. In this case complete settling occurs even
with large external bypass capacitors. The inputs (CH0 to
CH15, COM), on the other hand, are typically driven from
larger source resistances. Source resistances up to 10k
may interface directly to the LTC2498 and settle completely;
however, the addition of external capacitors at the input
terminals in order to filter unwanted noise (anti-aliasing)
results in incomplete settling.
The
LTC2498 offers two methods of removing these er-
rors. The first is an automatic differential input current
cancellation (Easy Drive) and the second is the insertion
of buffer between the MUXOUT and ADCIN pins, thus
isolating the input switching from the source resistance.
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 10k bridge driving a
0.1µF capacitor has a time constant an order of magnitude
greater than the required maximum.
The LTC2498 uses a proprietary switching algorithm
that forces the average differential input current to zero
Figure 12. LTC2498 Equivalent Analog Input Circuit
IN
+
IN
–
10k
INTERNAL
SWITCH
NETWORK
10k
C
EQ
12pF
10k
I
IN
–
REF
+
I
REF
+
I
IN
+
I
REF
–
2498 F12
SWITCHING FREQUENCY
f
SW
= 123kHz INTERNAL OSCILLATOR
f
SW
= 0.4 • f
EOSC
EXTERNAL OSCILLATOR
REF
–
10k
100Ω
INPUT
MULTIPLEXER
EXTERNAL
CONNECTION
100Ω
MUXOUTP ADCINP
EXTERNAL
CONNECTION
MUXOUTN ADCINN
I IN
+
( )
AVG
= I IN
–
( )
AVG
=
V
IN(CM)
− V
REF(CM)
0.5• R
EQ
I REF
+
( )
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
−
, WHERE IN
+
AND IN
−
ARE THE SELECTEDINPUT CHANNELS
V
IN(CM)
=
IN
+
–IN
−
2
⎛
⎝
⎜
⎜
⎞
⎠
⎟
⎟
R
EQ
= 2.71MΩINTERNAL OSCILLATOR 60Hz MODE
R
EQ
= 2.98MΩINTERNAL OSCILLATOR 50Hz/60Hz MODE
R
EQ
= 0.833• 10
12
( )
/f
EOSC
EXTERNAL OSCILLATOR