LTC2492 Linear Technology, LTC2492 Datasheet - Page 28

• Manufacturer Part Number: LTC2492
• Description: 24-Bit 2-/4-Channel ADC
• Manufacturer: Linear Technology

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APPLICATIONS INFORMATION
LTC2492
conversion cycle, the average differential input current
(I
zero, the common mode input current (I
proportional to the difference between the common mode
input voltage (V
voltage (V
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
common mode input currents 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
difference between V
common mode voltage of 2.5V and an input common mode
of 1.5V, the common mode input current is approximately
0.74μA. This common mode input current does not degrade
the accuracy if the source impedances tied to IN
IN
to a fi xed offset error but do not effect the linearity or full
scale reading. A 1% mismatch in a 1k source resistance
leads to a 74μV shift in offset voltage.
In applications where the common mode input voltage
varies as a function of the input signal level (single-ended
type sensors), the common mode input current varies
proportionally with input voltage. For the case of balanced
input impedances, the common mode input current effects
are rejected by the large CMRR of the LTC2492, leading to little
degradation in accuracy. Mismatches in source impedances
lead to gain errors proportional to the difference between
the common mode input and common mode reference.
1% mismatches in 1k source resistances lead to gain
errors on the order of 15ppm. Based on the stability of the
internal sampling capacitors and the accuracy of the internal
oscillator, a one-time calibration will remove this error.
In addition to the input sampling current, the input ESD
protection diodes have a temperature dependent leakage
current. This current, nominally 1nA (±10nA Max), results
in a small offset shift. A 1k source resistance will create a
1μV typical and a 10μV maximum offset voltage.
28
IN
–
+
are matched. Mismatches in source impedance lead
– I
IN
–
REF(CM)
) is zero. While the differential input current is
IN(CM)
).
IN(CM)
) and the common mode reference
and V
REF(CM)
. For a reference
IN
+
+ I
IN
–
+
)/2 is
and
Reference Current
Similar to the analog inputs, the LTC2492 samples the
differential reference pins (REF
small amounts of charge to and from these pins, thus
producing a dynamic reference current. If incomplete
settling occurs (as a function the reference source
resistance and reference bypass capacitance) linearity
and gain errors are introduced.
For relatively small values of external reference capacitance
(C
for reference impedances of many kΩ (if C
to 10kΩ will not degrade the performance) (see Figures
13 and 14).
REF
Figure 13. +FS Error vs R
Figure 14. –FS Error vs R
< 1nF), the voltage on the sampling capacitor settles
–40
–50
–60
–80
–90
–10
–20
–30
–70
10
0
–10
90
80
70
60
50
40
30
20
10
0
0
V
V
V
V
F
T
O
A
0
CC
REF
IN
IN
= GND
= 25°C
+
–
V
V
V
V
F
T
C
= 5V
O
A
CC
REF
IN
IN
= 1.25V
= 3.75V
= 5V
C
REF
C
= GND
= 25°C
+
–
REF
10
REF
= 5V
= 3.75V
= 1.25V
C
= 5V
C
= 0.001μF
REF
10
C
REF
= 0.01μF
C
= 100pF
REF
REF
C
= 0pF
= 0.001μF
REF
= 0.01μF
100
R
= 100pF
SOURCE
= 0pF
100
R
SOURCE
SOURCE
SOURCE
(Ω)
1k
(Ω)
1k
+
and REF
at V
at V
10k
10k
REF
REF
2492 F14
100k
(Small C
(Small C
2492 F13
REF
–
100k
) transferring
= 100pF up
REF
REF
)
)
2492fb