LTC2424IG Linear Technology, LTC2424IG Datasheet - Page 25

• Manufacturer Part Number: LTC2424IG
• Description: IC ADC 20BIT 4CH MICROPWR 28SSOP
• Manufacturer: Linear Technology

Specifications of LTC2424IG

• Number Of Bits: 20
• Sampling Rate (per Second): 7.5
• Data Interface: MICROWIRE™, Serial, SPI™
• Number Of Converters: 1
• Power Dissipation (max): 1mW
• Voltage Supply Source: Single Supply
• Operating Temperature: -40°C ~ 85°C
• Mounting Type: Surface Mount
• Package / Case: 28-SSOP (0.200", 5.30mm Width)
• Lead Free Status / RoHS Status: Contains lead / RoHS non-compliant

APPLICATIONS
While a digital input signal is in the 0.5V to (V
range, the CMOS input receiver draws additional current
from the power supply. It should be noted that, when any
one of the digital input signals (F
CLK and SCK in External SCK mode of operation) is within
this range, the LTC2424/LTC2428 power supply current
may increase even if the signal in question is at a valid logic
level. For micropower operation and in order to minimize
the potential errors due to additional ground pin current,
it is recommended to drive all digital input signals to full
CMOS levels [V
Severe ground pin current disturbances can also occur
due to the undershoot of fast digital input signals. Under-
shoot and overshoot can occur because of the imped-
ance mismatch at the converter pin when the transition
time of an external control signal is less than twice the
propagation delay from the driver to LTC2424/LTC2428.
For reference, on a regular FR-4 board, signal propaga-
tion velocity is approximately 183ps/inch for internal
traces and 170ps/inch for surface traces. Thus, a driver
generating a control signal with a minimum transition
time of 1ns must be connected to the converter pin
through a trace shorter than 2.5 inches. This problem
becomes particularly difficult when shared control lines
are used and multiple reflections may occur. The solution
is to carefully terminate all transmission lines close to
their characteristic impedance.
IL
SELECTED
CHANNEL
f
f
EXTERNAL OSCILLATOR: 2.56kHz f 307.2kHz
OUT
OUT
< 0.4V and V
CHX
U
= 50Hz, INTERNAL OSCILLATOR: f = 128kHz
= 60Hz, INTERNAL OSCILLATOR: f = 153.6kHz
I
I
IN(MUX)
IN(MUX)
INFORMATION
U
MUXV
(PIN 8)
OH
O
Figure 18. LTC2424/LTC2428 Equivalent Analog Input Circuit
CC
, CSADC, CSMUX, D
> (V
R
75
W
SW
CC
– 0.4V)].
MUXOUT
CC
U
– 0.5V)
ADCIN
ZS
I
DC
SET
IN
FS
,
SET
I
I
IN(LEAK)
IN(LEAK)
Parallel termination near the LTC2424/LTC2428 input
pins will eliminate this problem but will increase the driver
power dissipation. A series resistor between 27 and 56
placed near the driver or near the LTC2424/LTC2428 pin
will also eliminate this problem without additional power
dissipation. The actual resistor value depends upon the
trace impedance and connection topology.
Driving the Input and Reference
The analog input and reference of the typical delta-sigma
analog-to-digital converter are applied to a switched ca-
pacitor network. This network consists of capacitors switch-
ing between the analog input (ADCIN), ZS
the reference (FS
seen at both ADCIN and V
circuit is shown in Figure 18.
The key to understanding the effects of this dynamic input
current is based on a simple first order RC time constant
model. Using the internal oscillator, the internal switched
capacitor network of the LTC2424/LTC2428 is clocked at
153,600Hz corresponding to a 6.5 s sampling period.
Fourteen time constants are required each time a capacitor
is switched in order to achieve 1ppm settling accuracy.
Therefore, the equivalent time constant at V
should be less than 6.5 s/14 = 460ns in order to achieve
1ppm accuracy.
I
I
REF
REF
ADCV
ADCV
(PIN 2)
(PIN 2)
CC
CC
R
R
R
5k
5k
5k
SW
SW
SW
SET
24248 F18
I
DC
). The result is small current spikes
= 0.25(V
AVERAGE INPUT CURRENT:
LTC2424/LTC2428
REF
IN
. A simplified input equivalent
– 0.5 • V
C
1pF (TYP)
EQ
REF
) • f • C
EQ
SET
IN
(Pin 5) and
and V
25
REF