LTC2498 Linear Technology, LTC2498 Datasheet - Page 27

• Manufacturer Part Number: LTC2498
• Description: 24-Bit 8-/16-Channel ADC
• Manufacturer: Linear Technology

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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 simplifi ed 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 in-
put/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
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
MUXOUTP
MUXOUTN
CONNECTION
CONNECTION
EXTERNAL
EXTERNAL
ADCINP
ADCINN
Figure 12. LTC2498 Equivalent Analog Input Circuit
10k
10k
10k
10k
INTERNAL
NETWORK
SWITCH
2498 F12
terminals in order to fi lter unwanted noise (anti-aliasing)
results in incomplete settling.
The LTC2498 offers two methods of removing these errors.
The fi rst is an automatic differential input current cancella-
tion (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 requied
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 propriatary switching algorithm
that forces the average differential input current to zero
C
12pF
EQ
I IN
I REF
R
R
R
where:
V
V
V
V
( )
( )
REF
REF(CM)
IN
IN(CM)
EQ
EQ
EQ
= IN
+
= 2.71M INTERNAL OSCILLATOR 60Hz MODE
= 2.98M INTERNAL OSCILLATOR 50Hz/60Hz MODE
= 0.833• 10
= REF
+
AVG
(
=
+
AVG
=
= I IN
+
IN
IN , WHERE IN
REF
+
( )
1.5V
REF
– IN
2
+
–
12
– REF
2
REF
AVG
)
/f
EOSC
+ V
=
(
0.5 • R
V
+
IN(CM)
REF(CM)
EXTERNAL OSCILLATOR
AND IN ARE THE SELECTED INPUT CHANNELS
0.5•R
EQ
– V
V
REF(CM)
EQ
IN(CM)
)
–
V
REF
LTC2498
V
IN
• R
2
EQ
27
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