LTC2242IUP-12 Linear Technology, LTC2242IUP-12 Datasheet - Page 18

• Manufacturer Part Number: LTC2242IUP-12
• Description: IC,A/D CONVERTER,SINGLE,12-BIT,CMOS,LLCC,64PIN
• Manufacturer: Linear Technology

Other voltage ranges in between the pin selectable ranges
can be programmed with two external resistors as shown
in Figure 10. An external reference can be used by applying
its output directly or through a resistor divider to SENSE.
It is not recommended to drive the SENSE pin with a logic
device. The SENSE pin should be tied to the appropriate
level as close to the converter as possible. If the SENSE pin
LTC2242-12
APPLICATIO S I FOR ATIO
TIE TO V
TIE TO V
18
RANGE = 2 • V
CM
0.5V < V
DD
FOR 2V RANGE;
FOR 1V RANGE;
SENSE
SENSE
Figure 9. Equivalent Reference Circuit
1.25V
1µF
1µF
< 1V
FOR
Figure 10. 1.5V Range ADC
0.75V
8k
12k
1.25V
U
2.2µF
0.1µF
2.2µF
0.1µF
SENSE
REFHA
REFHB
REFLB
REFLA
V
CM
U
SENSE
2.2µF
1µF
V
LTC2242-12
CM
CONTROL
DETECT
2Ω
RANGE
AND
LTC2242-12
W
1.25V BANDGAP
DIFF AMP
224212 F10
REFERENCE
INTERNAL ADC
HIGH REFERENCE
INTERNAL ADC
LOW REFERENCE
1V
BUFFER
0.5V
U
224212 F09
is driven externally, it should be bypassed to ground as
close to the device as possible with a 1µF ceramic capacitor.
Input Range
The input range can be set based on the application. The
2V input range will provide the best signal-to-noise perfor-
mance while maintaining excellent SFDR. The 1V input
range will have better SFDR performance, but the SNR will
degrade by 5dB. See the Typical Performance Character-
istics section.
Driving the Encode Inputs
The noise performance of the LTC2242-12 can depend
on the encode signal quality as much as on the analog
input. The ENC
differentially, primarily for noise immunity from com-
mon mode noise sources. Each input is biased through
a 4.8k resistor to a 1.5V bias. The bias resistors set the
DC operating point for transformer coupled drive circuits
and can set the logic threshold for single-ended drive
circuits.
Any noise present on the encode signal will result in
additional aperture jitter that will be RMS summed with the
inherent ADC aperture jitter.
In applications where jitter is critical (high input frequen-
cies) take the following into consideration:
1. Differential drive should be used.
2. Use as large an amplitude as possible; if transformer
coupled use a higher turns ratio to increase the amplitude.
3. If the ADC is clocked with a sinusoidal signal, filter the
encode signal to reduce wideband noise.
4. Balance the capacitance and series resistance at both
encode inputs so that any coupled noise will appear at both
inputs as common mode noise. The encode inputs have a
common mode range of 1.2V to 2.0V. Each input may be
driven from ground to V
+
/ENC
–
inputs are intended to be driven
DD
for single-ended drive.
224212f