LTC2205-14 LINER [Linear Technology], LTC2205-14 Datasheet - Page 18

• Manufacturer Part Number: LTC2205-14
• Description: 14-Bit, 65Msps ADC
• Manufacturer: LINER [Linear Technology]

APPLICATIONS INFORMATION
Reference Operation
Figure 6 shows the LTC2205-14 reference circuitry con-
sisting of a 2.5V bandgap reference, a programmable gain
amplifi er and control circuit. The LTC2205-14 has three
modes of reference operation: Internal Reference, 1.25V
external reference or 2.5V external reference. To use the
internal reference, tie the SENSE pin to V
external reference, simply apply either a 1.25V or 2.5V
reference voltage to the SENSE input pin. Both 1.25V
and 2.5V applied to SENSE will result in a full-scale range
of 2.25V
for a common mode bias for input drive circuitry. An
external bypass capacitor is required for the V
This provides a high 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 capacitor. The minimum value required
for stability is 2.2µF.
The internal programmable gain amplifi er provides the
internal reference voltage for the ADC. This amplifi er has
very stringent settling requirements and is not accessible
for external use.
LTC2205-14
18
ANALOG
INPUT
Figure 5. DC Coupled Input with Differential Amplifi er
P-P
AMPLIFIER = LTC6600-20,
LTC1993, ETC.
(PGA = 0). A 1.25V output, V
DIFFERENTIAL
+
CM
–
HIGH SPEED
AMPLIFIER
+
–
25Ω
25Ω
2.2µF
12pF
12pF
V
A
A
CM
IN
IN
+
–
CM
DD
LTC2205-14
. To use an
is provided
CM
220514 F05
output.
The SENSE pin can be driven ±5% around the nominal 2.5V
or 1.25V external reference inputs. This adjustment range
can be used to trim the ADC gain error or other system
gain errors. When selecting the internal reference, the
SENSE pin should be tied to V
as possible. If the sense pin is driven externally it should
be bypassed to ground as close to the device as possible
with 1µF (or larger) ceramic capacitor.
TIE TO V
Figure 7. A 2.25V Range ADC with an External 2.5V Reference
EXTERNAL 1.25V
EXTERNAL 2.5V
INTERNAL 2.5V
OR INPUT FOR
OR INPUT FOR
REFERENCE
REFERENCE
REFERENCE
DD
TO USE
3.3V
1µF
SENSE
2.2µF
V
2
CM
Figure 6. Reference Circuit
LTC1461-2.5
LTC2205-14
1.25V
4
BUFFER
6
DD
AND GAIN
CONTROL
SELECT
RANGE
as close to the converter
SENSE
2.2µF
2.2µF
V
CM
1.25V
LTC2205-14
220514 F07
PGA
REFERENCE
BANDGAP
REFERENCE
INTERNAL
2.5V
ADC
220514fa
220514 F06