LTC2482 Datasheet, PDF (24/32 Pages) Linear Technology – 16-Bit ΔΣ ADC with Easy Drive Input Current Cancellation

LTC2482
APPLICATIONS INFORMATION
R_SOURCE
V_INCM + 0.5V_IN
C_IN
C_PAR
20pF
IN⁺
LTC2482
R_SOURCE
V_INCM – 0.5V_IN
C_IN
C_PAR
20pF
IN^–
2482 F11
Figure 11. An RC Network at IN⁺ and IN^–
80
V_CC = 5V
60
40
VVV^{RIINNE+–F}
=
=
=
5V
3.75V
1.25V
f_O = GND
20 ^{TA = 25°C}
C_IN = 0pF
C_IN = 100pF
0
–20
C_IN = 1nF, 0.1μF, 1μF
–40
–60
–80
1
10
100
1k
10k 100k
R_SOURCE (Ω)
2482 F12
Figure 12. +FS Error vs R_SOURCE at IN⁺ and IN^–
80
V_CC = 5V
60
40
VVV^{RIINNE+–F}
=
=
=
5V
1.25V
3.75V
f_O = GND
20 ^{TA = 25°C}
C_IN = 1nF, 0.1μF, 1μF
0
–20
C_IN = 100pF
C_IN = 0pF
–40
–60
–80
1
10
100
1k
10k 100k
R_SOURCE (Ω)
2482 F13
Figure 13. –FS Error vs R_SOURCE at IN⁺ and IN^–
24
The magnitude of the dynamic input current depends upon
the size of the very stable internal sampling capacitors and
upon the accuracy of the converter sampling clock. The
accuracy of the internal clock over the entire temperature
and power supply range is typically better than 0.5%. Such
a specification can also be easily achieved by an external
clock. When relatively stable resistors (50ppm/°C) are
used for the external source impedance seen by IN⁺ and
IN^–, the expected drift of the dynamic current and offset
will be insignificant (about 1% of their respective values
over the entire temperature and voltage range). Even for
the most stringent applications, a one-time calibration
operation may be sufficient.
In addition to the input sampling charge, 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 10μV maximum offset voltage.
Reference Current
In a similar fashion, the LTC2482 samples the differential
reference pins V_REF+ and GND transferring small amount
of charge to and from the external driving circuits thus
producing a dynamic reference current. This current does
not change the converter offset, but it may degrade the
gain and INL performance. The effect of this current can
be analyzed in two distinct situations.
For relatively small values of the external reference capaci-
tors (C_REF < 1nF), the voltage on the sampling capacitor
settles almost completely and relatively large values for
the source impedance result in only small errors. Such
values for C_REF will deteriorate the converter offset and
gain performance without significant benefits of reference
filtering and the user is advised to avoid them.
Larger values of reference capacitors (C_REF > 1nF) may be
required as reference filters in certain configurations. Such
capacitors will average the reference sampling charge and
the external source resistance will see a quasi constant
reference differential impedance.
In the following discussion, it is assumed the input and
reference common mode are the same. Using internal
oscillator (50Hz/60Hz rejection), the differential reference
2482fb