LTC2494CUHF-PBF LINER [Linear Technology], LTC2494CUHF-PBF Datasheet - Page 29

• Manufacturer Part Number: LTC2494CUHF-PBF
• Description: 16-Bit 8-/16-Channel ?? ADC with PGA and Easy Drive Input Current Cancellation
• Manufacturer: LINER [Linear Technology]

APPLICATIONS INFORMATION
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.
Automatic Offset Calibration of External Buffers/
Amplifi ers
In addition to the Easy Drive input current cancellation,
the LTC2494 enables an external amplifer to be inserted
between the multiplexer output and the ADC input (see
Figure 13). This is useful in applications where balanced
source impedances are not possible. One pair of external
buffers/amplifers can be shared between all 17 analog
inputs. The LTC2494 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
Figure 13. External Buffers Provide High Impedance Inputs and Amplifi er Offsets are Automatically Cancelled.
ANALOG
INPUTS
17
INPUT
MUX
3
2
6
5
–
+
–
+
1/2 LTC6078
1/2 LTC6078
LTC2494
multiplexer and the ADC, it is inside this correction loop.
This results in automatic offset correction and offset drift
removal of the external amplifer.
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/√Hz. The Easy Drive input technology
of the LTC2494 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 LTC2494 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 capaci-
tance (C
settles for reference impedances of many kΩ (if C
100pF up to 10k will not degrade the performance) (see
Figures 14, 15).
1
7
1k
1k
REF
< 1nF), the voltage on the sampling capacitor
EASY DRIVE
ΔΣ ADC
INPUTS
WITH
2494 F13
0.1μF
0.1μF
SDO
SCK
SDI
CS
+
and REF
LTC2494
–
) transferring
29
REF
2494fb
=