Datasheet

LTC2496
25
2496fb
applicaTions inForMaTion
Automatic Offset Calibration of External Buffers/
Amplifiers
In addition to the Easy Drive input current cancellation,
the LTC2496 enables an external amplifier to be inserted
between the multiplexer output and the ADC input, see
Figure 11. 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 LTC2496 preforms 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 LTC2496 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 LTC2496 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 kΩ (if C
REF
= 100pF up
to 10kΩ will not degrade the performance), see Figures
12 and 13.
Figure 11. External Buffers Provide High Impedance Inputs and
Amplifier Offsets are Automatically Cancelled.
0.1µF
0.1µF
–
+
–
+
1/2 LTC6078
1/2 LTC6078
1
2
3
5
6
7
∆∑ ADC
WITH
EASY DRIVE
INPUTS
INPUT
MUX
MUXOUTP MUXOUTN
17
2496 F11
LTC2496
ANALOG
INPUTS
SDI
SCK
SDO
CS
1k
1k
Figure 12. +FS Error vs R
SOURCE
at V
REF
(Small C
REF
)
Figure 13. –FS Error vs R
SOURCE
at V
REF
(Small C
REF
)
R
SOURCE
(Ω)
0
+FS ERROR (ppm)
50
70
90
10k
2496 F12
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
2496 F13
–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