LTC2497 Linear Technology, LTC2497 Datasheet - Page 22

• Manufacturer Part Number: LTC2497
• Description: 16-Bit 8-/16-Channel ADC
• Manufacturer: Linear Technology

www.DataSheet4U.com
LTC2497
APPLICATIONS INFORMATION
source impedances are not possible. One pair of external
buffers/amplifi ers can be shared between all 17 analog
inputs. The LTC2497 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 amplifi er 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 amplifi er.
The LTC6078 is an excellent amplifi er for this function.
It operates with supply voltages as low as 2.7V and its
noise level is 18nV/√ ⎯ H ⎯ z . The Easy Drive input technology
of the LTC2497 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
22
Figure 11. +FS Error vs R
–10
90
80
70
60
50
40
30
20
10
0
0
V
V
V
V
F
T
O
A
CC
REF
IN
IN
= GND
= 25°C
+
–
= 5V
= 3.75V
= 1.25V
= 5V
C
C
10
REF
C
REF
REF
C
= 0.001µF
REF
= 0.01µF
= 100pF
= 0pF
100
R
SOURCE
SOURCE
(Ω)
1k
at V
REF
10k
(Small C
2497 F11
100k
REF
)
can also be biased at supply rails beyond those used by
the LTC2497. This allows the external sensor to swing rail-
to-rail (–0.3V to V
level shift circuitry.
Reference Current
Similar to the analog inputs, the LTC2497 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).
REF
Figure 12. –FS Error vs R
< 1nF), the voltage on the sampling capacitor settles
–10
–20
–30
–40
–50
–60
–70
–80
–90
10
0
0
V
V
V
V
F
T
O
CC
REF
IN
IN
A
= GND
= 25°C
+
–
CC
C
= 5V
= 1.25V
= 3.75V
C
= 5V
REF
C
REF
10
REF
+ 0.3V) without the need of external
C
= 0.001µF
REF
= 0.01µF
= 100pF
= 0pF
100
R
SOURCE
SOURCE
+
(Ω)
1k
and REF
at V
REF
10k
(Small C
2497 F12
REF
–
) transferring
100k
= 100pF up
REF
)
2497f