LTC2488 LINER [Linear Technology], LTC2488 Datasheet - Page 22

• Manufacturer Part Number: LTC2488
• Description: 16-Bit 2-/4-Channel ?? ADC with Easy Drive Input Current Cancellation
• Manufacturer: LINER [Linear Technology]

APPLICATIONS INFORMATION
LTC2488
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 LTC2488, leading
to little degradation in accuracy. Mismatches in source
impedances lead to gain errors proportional to the dif-
ference 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 ac-
curacy 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.
Reference Current
Similar to the analog inputs, the LTC2488 samples the
differential reference pins (REF
small amounts of charge to and from these pins, thus
producing a dynamic reference current. If incomplete set-
tling 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
11 and 12).
In cases where large bypass capacitors are required on
the reference inputs (C
ity errors are proportional to the value of the reference
resistance. Every ohm of reference resistance produces
a full-scale error of approximately 0.5ppm (while operat-
22
–
REF
are matched. Mismatches in source impedance lead
< 1nF), the voltage on the sampling capacitor settles
REF
> 0.01µF) full-scale and linear-
+
and REF
REF
–
) transferring
= 100pF up
ing with the internal oscillator) (see Figures 13 and 14). If
the input common mode voltage is equal to the reference
common mode voltage, a linearity error of approximately
0.67ppm per 100Ω of reference resistance results (see
Figure 15). In applications where the input and reference
common mode voltages are different, the errors increase.
A 1V difference in between common mode input and
common mode reference results in a 6.7ppm INL error
for every 100Ω of reference resistance.
In addition to the reference sampling charge, the reference
ESD protection diodes have a temperature dependent leak-
age current. This leakage current, nominally 1nA (±10nA
max) results in a small gain error. A 100Ω reference
resistance will create a 0.5µV full scale error.
Figure 11. +FS Error vs R
Figure 12. –FS Error vs R
–10
–10
–20
–30
–40
–50
–60
–70
–80
–90
90
80
70
60
50
40
30
20
10
10
0
0
0
0
V
V
V
V
F
T
V
V
V
V
F
T
O
O
A
A
CC
REF
IN
IN
CC
REF
IN
IN
= GND
= 25°C
= GND
= 25°C
+
–
+
–
C
= 5V
= 5V
= 1.25V
= 3.75V
= 3.75V
= 1.25V
C
= 5V
REF
= 5V
C
C
REF
C
10
10
REF
REF
C
REF
C
REF
= 0.001µF
REF
C
= 0.01µF
= 0.001µF
= 100pF
REF
= 0.01µF
= 100pF
= 0pF
= 0pF
100
R
100
R
SOURCE
SOURCE
SOURCE
SOURCE
(Ω)
(Ω)
1k
1k
at V
at V
10k
10k
REF
REF
2488 F11
2488 F12
(Small C
(Small C
100k
100k
REF
REF
)
)
2488f