ltc2484 Linear Technology Corporation, ltc2484 Datasheet - Page 31

• Manufacturer Part Number: ltc2484
• Description: 24-bit Delta Sigma Adc With Easy Drive Input Current Cancellation
• Manufacturer: Linear Technology Corporation

APPLICATIO S I FOR ATIO
external conversion clock (F
oscillator), the LTC2484 output data rate can be increased
as desired. The duration of the conversion phase is 41036/
f
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 rely-
ing upon the LTC2484’s exceptional common mode rejec-
tion and by carefully eliminating common mode to differ-
ential mode conversion sources in the input circuit. The
user should avoid single-ended input filters and should
maintain 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 transferred
through the input and the reference pins. If large external
input and/or reference capacitors (C
previous section provides formulae for evaluating the
effect of the source resistance upon the converter perfor-
mance for any value of f
reference capacitors (C
external source resistance upon the LTC2484 typical
performance can be inferred from Figures 13, 14, 15 and
16 in which the horizontal axis is scaled by 307200/f
Third, an increase in the frequency of the external oscilla-
tor above 1MHz (a more than 3× 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 20 to 27.
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
EOSC
. If f
EOSC
= 307.2kHz, the converter behaves as if the
EOSC
EOSC
U
will result in a proportional change
EOSC
over the nominal 307.2kHz will
IN
, C
U
+
and IN
REF
. If small external input and/or
O
) are used, the effect of the
connected to an external
–
W
pins.
IN
, C
REF
) are used, the
U
EOSC
.
Figure 20. Offset Error vs Output Data Rate and Temperature
Figure 21. +FS Error vs Output Data Rate and Temperature
Figure 22. –FS Error vs Output Data Rate and Temperature
–1000
–1500
–2000
–2500
–3000
–3500
–500
3500
3000
2500
2000
1500
1000
–10
500
50
40
30
20
10
0
0
0
0
0
0
V
V
F
V
V
V
F
V
V
F
O
O
O
IN(CM)
CC
IN
IN(CM)
CC
IN(CM)
CC
10
10
10
OUTPUT DATA RATE (READINGS/SEC)
= EXT CLOCK
OUTPUT DATA RATE (READINGS/SEC)
= EXT CLOCK
OUTPUT DATA RATE (READINGS/SEC)
= EXT CLOCK
= 0V
= V
= V
= V
20 30
20 30
20
REF
REF
REF
= V
= V
= V
REF(CM)
REF(CM)
= 5V
REF(CM)
= 5V
= 5V
30
T
T
T
A
40
T
40
40
A
A
A
= 85°C
= 85°C
= 25°C
= 85°C
50
50
50
60
60
60
T
A
70
70
70
T
= 25°C
A
= 25°C
80
80
80
90
90 100
90 100
2484 F22
2484 F20
2484 F21
LTC2484
100
31
2484fa