LTC2440 Linear Technology, LTC2440 Datasheet - Page 22

• Manufacturer Part Number: LTC2440
• Description: 24-Bit High Speed Differential delta-sigma ADC
• Manufacturer: Linear Technology

LTC2440
APPLICATIO S I FOR ATIO
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 simplified equivalent circuit is shown in
Figure 16.
The average input and reference currents can be ex-
pressed in terms of the equivalent input resistance of the
sample capacitor, where:
22
CONVERTER
Req = 1/(f
CURRENT
SUPPLY
STATE
CS
SW
• Ceq)
SLEEP
8µA
U
U
DATA
OUT
W
8mA
CONVERT
Figure 15. Reduced Power Timing Mode
SWITCHING FREQUENCY
f
f
Figure 16. LTC2440 Input Structure
SW
SW
V
V
I
V
V
I
REF
REF
I
I
REF
REF
IN
IN
IN
IN
= 1.8MHz INTERNAL OSCILLATOR
= f
+
+
–
–
+
+
–
–
U
EOSC
V
V
CC
CC
/5 EXTERNAL OSCILLATOR
V
V
I
I
I
I
LEAK
LEAK
LEAK
LEAK
CC
CC
I
I
I
I
LEAK
LEAK
LEAK
LEAK
When using the internal oscillator, f
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 reflections 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.
R
R
R
R
SW
SW
SW
SW
500Ω
500Ω
500Ω
500Ω
(TYP)
(TYP)
(TYP)
(TYP)
SLEEP
8µA
2440 F16
C
5pF
(TYP)
(C
EQ
EQ
= 3.5pF SAMPLE CAP + PARASITICS)
DATA
OUT
8mA
CONVERT
SW
is 1.8MHz and the
8µA
sn2440, 2440fas
SLEEP
2440 F15