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LTC2494
2494fd
applications inForMation
larger source resistances. Source resistances up to 10k
mayinterfacedirectlytotheLTC2494andsettlecompletely;
however, the addition of external capacitors at the input
terminals in order to filter unwanted noise (anti-aliasing)
results in incomplete settling.
TheLTC2494offerstwomethodsofremovingtheseerrors.
The first is 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 10k
Ω with no external bypass capacitor or up
to 500
Ω 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 bridge driving a
0.1F capacitor has a time constant an order of magnitude
greater than the required maximum.
The LTC2494 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
of buffers.
The switching algorithm forces the average input current
on the positive input (IIN+) to be equal to the average input
current in the negative input (IIN–). Over the complete
conversion cycle, the average input current (IIN+ – IIN–)
is zero. While the differential input current is zero, the
common mode input current (IIN+ + IIN–)/2 is proportional
to the difference between the common mode input volt-
age (VIN(CM)) and the common mode reference voltage
(VREF(CM)).
In applications where the input common mode voltage is
equal to the reference common mode voltage, as in the
case of a balanced bridge, both the differential and com-
mon mode input current are zero. The accuracy of the
converter is not compromised by settling errors.
In applications where the input common mode voltage is
constant but different from the reference common mode
voltage, the differential input current remains zero while
the common mode input current is proportional to the
Figure 12. LTC2494 Equivalent Analog Input Circuit
IN+
IN–
10k
INTERNAL
SWITCH
NETWORK
10k
CEQ
12pF
10k
IIN–
REF+
IREF+
IIN+
IREF–
2494 F12
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
I IN+
( )
AVG
=
I IN–
( )
AVG
=
VIN(CM) VREF(CM)
0.5REQ
I REF+
(
)
AVG
≈
1.5VREF + VREF(CM) – VIN(CM)
(
)
0.5 REQ
–
VIN2
VREF REQ
where:
VREF =REF
+ REF
VREF(CM) =
REF+ – REF
2
VIN =IN
+ IN, WHERE IN+ ANDIN ARE THE SELECTEDINPUT CHANNELS
VIN(CM) =
IN+ –IN
2
REQ =2.71MΩINTERNAL OSCILLATOR 60Hz MODE
REQ =2.98MΩINTERNAL OSCILLATOR 50Hz/60Hz MODE
REQ = 0.8331012
(
)/fEOSC EXTERNAL OSCILLATOR
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