Datasheet

LTC2498
30
2498ff
applications inForMation
The LTC6078 is an excellent amplifier for this function. It
operates with supply voltages as low as 2.7V and its noise
level is 18nV/√Hz. The Easy Drive input technology of the
LTC2498 enables an RC network to be added directly to
the output of the LTC6078. The capacitor reduces the
magnitude of the current spikes seen at the input to the
ADC and the resistor isolates the capacitor load from the
op-amp output enabling stable operation.
Reference Current
Similar to the analog inputs, the LTC2498 samples the
differential reference pins (REF
+
and REF
–
) transferring
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 capaci-
tance (C
REF
< 1nF), the voltage on the sampling capacitor
settles for reference impedances of many kΩ (if C
REF
=
100pF up to 10k will not degrade the performance), see
Figures 14, 15.
In cases where large bypass capacitors are required on
the reference inputs (C
REF
> 0.01µF) full-scale and linear-
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-
ing in simultaneous 50Hz/60Hz mode), see Figures 16
and 17. 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 18. 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 projection 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.
Normal Mode Rejection and Anti-Aliasing
One of the advantages delta-sigma ADCs offer over
conventional ADCs is on-chip digital filtering. Combined
with a large oversample ratio, the LTC2498 significantly
simplifies anti-aliasing filter requirements. Additionally,
the input current cancellation feature allows external
lowpass filtering without degrading the DC performance
of the device.
Figure 14. +FS Error vs R
SOURCE
at V
REF
(Small C
REF
)
Figure 15. –FS Error vs R
SOURCE
at V
REF
(Small C
REF
)
R
SOURCE
(Ω)
0
+FS ERROR (ppm)
50
70
90
10k
2498 F14
30
10
40
60
80
20
0
–10
10
100
1k
100k
V
CC
= 5V
V
REF
= 5V
V
IN
+
= 3.75V
V
IN
–
= 1.25V
f
O
= GND
T
A
= 25°C
C
REF
= 0.01µF
C
REF
= 0.001µF
C
REF
= 100pF
C
REF
= 0pF
R
SOURCE
(Ω)
0
–FS ERROR (ppm)
–30
–10
10
10k
2498 F15
–50
–70
–40
–20
0
–60
–80
–90
10
100
1k
100k
V
CC
= 5V
V
REF
= 5V
V
IN
+
= 1.25V
V
IN
–
= 3.75V
f
O
= GND
T
A
= 25°C
C
REF
= 0.01µF
C
REF
= 0.001µF
C
REF
= 100pF
C
REF
= 0pF