LTC2485 LINER [Linear Technology], LTC2485 Datasheet - Page 26

• Manufacturer Part Number: LTC2485
• Description: 24-Bit ?? ADC with Easy Drive Input Current Cancellation and I2C Interface
• Manufacturer: LINER [Linear Technology]

APPLICATIO S I FOR ATIO
LTC2485
phase is 41036/f
behaves as if the internal oscillator is used and the notch
is set at 60Hz.
An increase in f
translate into a proportional increase in the maximum
output data rate. The increase in output rate is neverthe-
less accompanied by three potential effects, which must
be carefully considered.
First, a change in f
in the internal notch position and in a reduction of the
converter differential mode rejection at the power line
frequency. In many applications, the subsequent perfor-
mance degradation can be substantially reduced by relying
upon the LTC2485’s exceptional common mode rejection
and by carefully eliminating common mode to differential
mode conversion sources in the input circuit. The user
should avoid single-ended input filters and should main-
tain a very high degree of matching and symmetry in the
circuits driving the IN
Second, the increase in clock frequency will increase
proportionally the amount of sampling charge trans-
ferred through the input and the reference pins. If large
external input and/or reference capacitors (C
used, the previous section provides formulae for evaluat-
ing the effect of the source resistance upon the converter
performance for any value of f
and/or reference capacitors (C
26
Figure 21. Offset Error vs Output Data Rate and Temperature
–10
50
40
30
20
10
0
0
V
V
V
CA0/F
EOSC
IN(CM)
CC
IN
EOSC
10
EOSC
OUTPUT DATA RATE (READINGS/SEC)
= 0V
= V
U
0
20
REF
= EXT CLOCK
= V
. If f
+
will result in a proportional change
REF(CM)
over the nominal 307.2kHz will
and IN
= 5V
30
U
EOSC
T
T
40
A
A
= 85°C
= 25°C
50
EOSC
–
= 307.2kHz, the converter
pins.
60
IN
. If small external input
, C
W
70
REF
80
) are used, the
90
2485 F21
100
IN
U
, C
REF
) are
effect of the external source resistance upon the LTC2485
typical performance can be inferred from Figures 14, 15,
16 and 17 in which the horizontal axis is scaled by
307200/f
Third, an increase in the frequency of the external oscillator
above 1MHz (a more than 3X increase in the output data
rate) will start to decrease the effectiveness of the internal
autocalibration circuits. This will result in a progressive
degradation in the converter accuracy and linearity. Typi-
cal measured performance curves for output data rates up
to 100 readings per second are shown in Figures 21 to 28.
In order to obtain the highest possible level of accuracy
from this converter at output data rates above 20 readings
per second, the user is advised to maximize the power
supply voltage used and to limit the maximum ambient
operating temperature. In certain circumstances, a reduc-
tion of the differential reference voltage may be beneficial.
Input Bandwidth
The combined effect of the internal SINC
of the analog and digital autocalibration circuits deter-
mines the LTC2485 input bandwidth. When the internal
oscillator is used with the notch set at 60Hz, the 3dB input
bandwidth is 3.63Hz. When the internal oscillator is used
with the notch set at 50Hz, the 3dB input bandwidth is
3.02Hz. If an external conversion clock generator of fre-
quency f
bandwidth is 11.8 • 10
Figure 22. +FS Error vs Output Data Rate and Temperature
EOSC
EOSC
3500
3000
2000
1000
2500
1500
500
0
is connected to the CA0/F
.
0
V
V
CA0/F
IN(CM)
CC
10
OUTPUT DATA RATE (READINGS/SEC)
= V
0
20 30
= EXT CLOCK
REF
= V
–6
REF(CM)
= 5V
• f
T
40
A
EOSC
= 85°C
50
60
.
70
T
A
= 25°C
80
0
pin, the 3dB input
4
90 100
2485 F22
digital filter and
2485fa