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LTC2495
2495fd
Driving the Input and Reference
The input and reference pins of the LTC2495 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 11.
When using the LTC2495’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-CH15, COM), on the other hand, are typically driven
from larger source resistances. Source resistances up
to 10k may interface directly to the LTC2495 and settle
completely; however, the addition of external capacitors
Figure 11. Equivalent Analog Input Circuit
at the input terminals in order to filter unwanted noise
(anti-aliasing) results in incomplete settling.
TheLTC2495offerstwomethodsofremovingtheseerrors.
Thefirstisautomaticdifferentialinputcurrentcancellation
(Easy Drive) and the second is the insertion of an external
bufferbetweentheMUXOUTandADCINpins,thusisolating
the input switching from the source resistance.
Automatic Differential Input Current Cancellation
In applications where the sensor output impedance is
low (up to 10k
W with no external bypass capacitor or up
to 500
W with 0.001F 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
W bridge driving a
0.1F capacitor has a time constant an order of magnitude
greater than the required maximum.
The LTC2495 uses a proprietary switching algorithm
that forces the average differential input current to zero
applications inForMation
I IN
V
R
I REF
AVG
IN CM
REF CM
EQ
+
( ) = ( ) =
–
(
)
(
)
.
0 5
++
( ) ≈
+
(
)
AVG
REF
REF CM
IN CM
EQ
IN
V
R
V
1 5
0 5
2
.
–
.
–
(
)
(
)
RREF
EQ
REF
REF CM
R
where
V
REF
V
REF
:
–
(
)
=
=
+
+
2
=
+
+
V
IN
IN WHERE IN ANDIN ARE THE SELECT
IN
,
EEDINPUT CHANNELS
V
IN
R
IN CM
EQ
(
)
–
.
=
=
+
2
2 711
60
M INTERNAL OSCILLATOR Hz MODE
W
R
2.98M INTERNA
EQ =
W
LL OSCILLATOR 50Hz/60Hz MODE
R
0.833 10
/f
EQ
12
EOS
=
(
)
CC EXTERNAL OSCILLATOR
IN+
IN–
10k
INTERNAL
SWITCH
NETWORK
10k
CEQ
12F
10k
IIN–
REF+
IREF+
IIN+
IREF–
2495 F11
SWITCHING FREQUENCY
fSW = 123kHz INTERNAL OSCILLATOR
fSW = 0.4 fEOSC EXTERNAL OSCILLATOR
REF–
10k
100
INPUT
MULTIPLEXER
EXTERNAL
CONNECTION
100
MUXOUTP
ADCINP
EXTERNAL
CONNECTION
MUXOUTN
ADCINN
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