LTC2497 Linear Technology, LTC2497 Datasheet - Page 20

• Manufacturer Part Number: LTC2497
• Description: 16-Bit 8-/16-Channel ADC
• Manufacturer: Linear Technology

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LTC2497
APPLICATIONS INFORMATION
of charge is transferred. A simplifi ed equivalent circuit is
shown in Figure 9.
When using the LTC2497’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 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-CH15, COM), on the other hand, are typically driven
from larger source resistances. Source resistances up
to 10k may interface directly to the LTC2497 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.
The LTC2497 offers two methods of removing these
errors. The fi rst is an automatic differential input current
cancellation (Easy Drive) and the second is the insertion
of an external buffer between the MUXOUT and ADCIN
pins, thus isolating the input switching from the source
resistance.
20
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
ADCINN
ADCINP
Figure 9. Equivalent Analog Input Circuit
10kΩ
10kΩ
10kΩ
10kΩ
INTERNAL
NETWORK
SWITCH
2497 F09
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 LTC2497 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
for buffers.
The switching algorithm forces the average input current
on the positive input (I
current on the negative input (I
conversion cycle, the average differential input current
(I
IN
C
12µF
EQ
+
– I
IN
–
I IN
I I REF
R
R
where
V
V
V
V
( )
( )
REF
REF CM
IN
IN CM
EQ
E E Q
) is zero. While the differential input current is
(
+
=
(
=
=
=
IN
+
:
)
(
AVG
2.98M INTERNAL OSCILLATOR
REF
0.833 10
+
=
) )
AVG
=
⎛
⎜
⎜
⎝
−
=
⎛
⎜
⎜
⎝
IN
+
IN WHERE IN AN
≈
REF
I IN
−
Ω
+
•
−
( )
1 5
REF
,
– – IN
2
.
–
+
12
V
–
2
REF
−
−
AVG
)
IN
REF
⎞
⎟
⎟
⎠
/f
EOSC
+
+
=
−
) to be equal to the average input
(
0 5
V
V
+
. •
⎞
⎟
⎟
⎠
IN CM
REF CM
EXTERNAL OSCILLATOR
(
R
. 0 5
(
D D IN ARE THE SELECTED INPUT CHANNELS
E E Q
)
−
)
•
–
V
R
−
REF CM
EQ
V
IN
IN CM
(
(
–
). Over the complete
)
)
)
–
V
REF
V
IN
•
2
R
EQ
2497f