Datasheet

LTC2205-14
18
220514fb
Figure 7. A 2.25V Range ADC with an External 2.5V Reference
Reference Operation
Figure 6 shows the LTC2205-14 reference circuitry
consisting 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
DD
. To
use an 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
P-P
(PGA = 0). A 1.25V output, V
CM
is provided
for a common mode bias for input drive circuitry. An
external bypass capacitor is required for the V
CM
output.
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.
APPLICATIONS INFORMATION
PGA
1.25V
SENSE
V
CM
BUFFER
INTERNAL
ADC
REFERENCE
RANGE
SELECT
AND GAIN
CONTROL
2.5V
BANDGAP
REFERENCE
2.2μF
TIE TO V
DD
TO USE
INTERNAL 2.5V
REFERENCE
OR INPUT FOR
EXTERNAL 2.5V
REFERENCE
OR INPUT FOR
EXTERNAL 1.25V
REFERENCE
220514 F06
LTC2205-14
Figure 6. Reference Circuit
V
CM
SENSE
1.25V
3.3V
2.2μF
2.2μF
1μF
220514 F07
LTC2205-14
LTC1461-2.5
2
6
4
Figure 5. DC Coupled Input with Differential Amplifi er
–
–
+
+
A
IN
+
A
IN
–
2.2μF
12pF
12pF
V
CM
LTC2205-14
ANALOG
INPUT
220514 F05
CM
AMPLIFIER = LTC6600-20,
LTC1993, ETC.
HIGH SPEED
DIFFERENTIAL
AMPLIFIER
25Ω
25Ω
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
DD
as close to 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 1μF (or larger) ceramic capacitor.