Datasheet

LTC2440
22
2440fd
APPLICATIONS INFORMATION
LTC2440 Input Structure
Modern delta sigma converters have switched capacitor
front ends that repeatedly sample the input voltage over
some time period. The sampling process produces a small
current pulse at the input and reference terminals as the
capacitors are charged. The LTC2440 switches the input
and reference to a 5pF sample capacitor at a frequency
of 1.8MHz. A simplifi ed equivalent circuit is shown in
Figure 16.
The average input and reference currents can be expressed
in terms of the equivalent input resistance of the sample
capacitor, where:
Req = 1/(f
SW
• Ceq)
When using the internal oscillator, f
SW
is 1.8MHz and the
equivalent resistance is approximately 110kΩ.
Driving the Input and Reference
Because of the small current pulses, excessive lead length
at the analog or reference input may allow refl ections or
ringing to occur, affecting the conversion accuracy. The
key to preserving the accuracy of the LTC2440 is complete
settling of these sampling glitches at both the input and
reference terminals. There are several recommended
methods of doing this.
Figure 15. Reduced Power Timing Mode
Figure 16. LTC2440 Input Structure
SLEEP CONVERT SLEEP CONVERT SLEEP
8mA
2440 F15
8μA8μA8mA8μA
DATA
OUT
DATA
OUT
CONVERTER
STATE
SUPPLY
CURRENT
CS
V
REF
+
V
IN
+
V
CC
R
SW
(TYP)
500Ω
I
LEAK
I
LEAK
V
CC
I
LEAK
I
LEAK
V
CC
R
SW
(TYP)
500Ω
C
EQ
5pF
(TYP)
(C
EQ
= 3.5pF SAMPLE CAP + PARASITICS)
R
SW
(TYP)
500Ω
I
LEAK
I
IN
+
V
IN
–
I
IN
–
I
REF
+
I
REF
–
2440 F16
I
LEAK
V
CC
I
LEAK
I
LEAK
SWITCHING FREQUENCY
f
SW
= 1.8MHz INTERNAL OSCILLATOR
f
SW
= f
EOSC
/5 EXTERNAL OSCILLATOR
V
REF
–
R
SW
(TYP)
500Ω