LTC1743 Linear Technology, LTC1743 Datasheet - Page 16

• Manufacturer Part Number: LTC1743
• Description: 12-Bit 50Msps ADC
• Manufacturer: Linear Technology

TIE TO V
LTC1743
APPLICATIO S I FOR ATIO
An external bypass capacitor of 4.7 F or larger is required
for the 2.5V reference output, V
frequency low impedance path to ground for internal and
external circuitry. This is also the compensation capacitor
for the reference. It will not be stable without this capaci-
tor.
The difference amplifier generates the high and low refer-
ence for the ADC. High speed switching circuits are
connected to these outputs and they must be externally
bypassed. Each output has two pins: REFHA and REFHB
for the high reference and REFLA and REFLB for the low
reference. The doubled output pins are needed to reduce
package inductance. Bypass capacitors must be con-
nected as shown in Figure 5.
Other voltage ranges in between the pin selectable ranges
can be programmed with two external resistors as shown
in Figure 6a. 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 since the logic threshold is close to ground and
V
16
TIE TO GND FOR 2V RANGE;
DD
RANGE = 2 • V
. The SENSE pin should be tied high or low as close to
DD
1V < V
FOR 3.2V RANGE;
SENSE
SENSE
Figure 5. Equivalent Reference Circuit
< 1.6V
2.5V
FOR
1 F
1 F
U
4.7 F
0.1 F
4.7 F
0.1 F
SENSE
REFHA
REFHB
REFLB
REFLA
V
U
CM
LTC1743
CONTROL
4
DETECT
RANGE
CM
AND
. This provides a high
W
2.5V BANDGAP
DIFF AMP
REFERENCE
INTERNAL ADC
HIGH REFERENCE
INTERNAL ADC
LOW REFERENCE
1.6V
U
BUFFER
1V
1743 F05
the converter as possible. If the SENSE pin is driven
externally, it should be bypassed to ground as close to the
device as possible with a 1 F ceramic capacitor.
Input Range
The input range can be set based on the application. For
oversampled signal processing in which the input fre-
quency is low (<10MHz), the largest input range will
provide the best signal-to-noise performance while main-
taining excellent SFDR. For high input frequencies
(>10MHz), the 2V range will have the best SFDR perfor-
mance but the SNR will degrade by 1.5dB. See the Typical
Performance Characteristics section.
Driving the Encode Inputs
The noise performance of the LTC1743 can depend on the
encode signal quality as much as on the analog input. The
ENC/ENC inputs are intended to be driven differentially,
primarily for noise immunity from common mode noise
sources. Each input is biased through a 6k resistor to a 2V
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.
Figure 6b. 2.5V Range ADC with an External Reference
5V
0.1 F
4
Figure 6a. 2.2V Range ADC
LT1790-1.25
14k
11k
2.5V
1.1V
1, 2
6
SENSE
4.7 F
1 F
V
2.5V
1.25V
CM
LTC1743
www.DataSheet4U.com
SENSE
4.7 F
1 F
1743 F06a
V
CM
LTC1743
1743 F06b
1743f