Datasheet

LTC2485
26
2485fc
APPLICATIONS INFORMATION
with an external conversion clock (CA0/f
0
connected to
an external oscillator), the LTC2485 output data rate can
be increased as desired. The duration of the conversion
phase is 41036/f
EOSC
. If f
EOSC
= 307.2kHz, the converter
behaves as if the internal oscillator is used and the notch
is set at 60Hz.
An increase in f
EOSC
over the nominal 307.2kHz will
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
EOSC
will result in a proportional change
in the internal notch position and in a reduction of the
converter differential mode rejection at the power line fre-
quency. In many applications, the subsequent performance
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 fi lters and should maintain a
very high degree of matching and symmetry in the circuits
driving the IN
+
and IN
–
pins.
Second, the increase in clock frequency will increase
proportionally the amount of sampling charge transferred
through the input and the reference pins. If large external
input and/or reference capacitors (C
IN
, C
REF
) are used,
the previous section provides formulae for evaluating
the effect of the source resistance upon the converter
performance for any value of f
EOSC
. If small external input
and/or reference capacitors (C
IN
, C
REF
) are used, the ef-
fect 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
EOSC
.
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. Typical
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 reduction of the
differential reference voltage may be benefi cial.
Input Bandwidth
The combined effect of the internal SINC
4
digital fi lter and
of the analog and digital autocalibration circuits determines
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 frequency f
EOSC
is connected to the CA0/f
0
pin, the 3dB input bandwidth
is 11.8 • 10
–6
• f
EOSC
.
OUTPUT DATA RATE (READINGS/SEC)
–10
OFFSET ERROR (ppm OF V
REF
)
10
30
50
0
20
40
20 40 60 80
2485 F21
10010030507090
V
IN(CM)
= V
REF(CM)
V
CC
= V
REF
= 5V
V
IN
= 0V
CA0/f
0
= EXT CLOCK
T
A
= 85°C
T
A
= 25°C
OUTPUT DATA RATE (READINGS/SEC)
0
0
+FS ERROR (ppm OF V
REF
)
500
1500
2000
2500
3500
10
50
70
2485 F22
1000
3000
40
90
100
20
30
60 80
V
IN(CM)
= V
REF(CM)
V
CC
= V
REF
= 5V
CA0/f
0
= EXT CLOCK
T
A
= 85°C
T
A
= 25°C
Figure 21. Offset Error vs Output Data Rate and Temperature Figure 22. +FS Error vs Output Data Rate and Temperature