Datasheet

LTC2495
26
2495fd
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.
Automatic Offset Calibration of External Buffers/
Amplifiers
In addition to the Easy Drive input current cancellation,
the LTC2495 allows an external amplifier to be inserted
between the multiplexer output and the ADC input (see
Figure 12). This is useful in applications where balanced
source impedances are not possible. One pair of external
buffers/amplifiers can be shared between all 17 analog
inputs. The LTC2495 performs an internal offset calibration
every conversion cycle in order to remove the offset and
drift of the ADC. This calibration is performed through a
combination of front-end switching and digital process-
ing. Since the external amplifier is placed between the
multiplexer and the ADC, it is inside this correction loop.
This results in automatic offset correction and offset drift
removal of the external amplifier.
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 LTC2495 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. The LTC6078
can also be biased at supply rails beyond those used by
the LTC2495. This allows the external sensor to swing rail-
to-rail (–0.3V to V
CC
+ 0.3V) without the need of external
level-shift circuitry.
Reference Current
Similar to the analog inputs, the LTC2495 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 capacitance
(C
REF
< 1nF), the voltage on the sampling capacitor settles
for reference impedances of many kW (if C
REF
= 100pF up
to 10kW will not degrade the performance (see Figures
13 and 14)).
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 15
and 16)). If the input common mode voltage is equal to
the reference common mode voltage, a linearity error of
approximately 0.67ppm per 100W of reference resistance
Figure 13. +FS Error vs R
SOURCE
at V
REF
(Small C
REF
) Figure 14. –FS Error vs R
SOURCE
at V
REF
(Small C
REF
)
applications inForMation
R
SOURCE
(Ω)
0
+FS ERROR (ppm)
50
70
90
10k
2495 F13
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
2495 F14
–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