Datasheet

LTC2499
24
2499fd
Driving the Input and Reference
The input and reference pins of the LTC2499 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 LTC2499’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 LTC2499 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 LTC2499 offers two methods of removing these
errors. The first 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.
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 LTC2499 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
IN
+
) to be equal to the average input
current on the negative input (I
IN
–
). Over the complete
conversion cycle, the average differential input current
applications inForMation
Figure 11. Equivalent Analog Input Circuit
IN
+
IN
–
10kΩ
INTERNAL
SWITCH
NETWORK
10kΩ
C
EQ
12µF
10kΩ
I
IN
–
REF
+
I
REF
+
I
IN
+
I
REF
–
2499 F11
SWITCHING FREQUENCY
f
SW
= 123kHz INTERNAL OSCILLATOR
f
SW
= 0.4 • f
EOSC
EXTERNAL OSCILLATOR
REF
–
10kΩ
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