LTC2205 Linear Technology, LTC2205 Datasheet - Page 23

• Manufacturer Part Number: LTC2205
• Description: (LTC2204 / LTC2205) 65Msps/40Msps
• Manufacturer: Linear Technology

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APPLICATIONS INFORMATION
Direct Coupled Circuits
Figure 5 demonstrates the use of a differential amplifi er to
convert a single ended input signal into a differential input
signal. The advantage of this method is that it provides
low frequency input response; however, the limited gain
bandwidth of any op amp or closed-loop amplifi er will de-
grade the ADC SFDR at high input frequencies. Additionally,
wideband op amps or differential amplifi ers tend to have
high noise. As a result, the SNR will be degraded unless
the noise bandwidth is limited prior to the ADC input.
Reference Operation
Figure 6 shows the LTC2205/LTC2204 reference circuitry
consisting of a 2.5V bandgap reference, a programmable
gain amplifi er and control circuit. The LTC2205/LTC2204
have three modes of reference operation: Internal Refer-
ence, 1.25V external reference or 2.5V external reference.
To use the internal reference, tie the SENSE pin to V
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
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
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
LTC2205/
LTC2204
is provided
CM
22054 F05
output.
DD
. To
very stringent settling requirements and is not accessible
for external use.
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
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
Figure 7. A 2.25V Range ADC with
an External 2.5V Reference
2.2µF
V
CM
Figure 6. Reference Circuit
2
LT1461-2.5
1.25V
LTC2205/
LTC2204
LTC2205/LTC2204
4
BUFFER
6
DD
AND GAIN
CONTROL
SELECT
RANGE
SENSE
as close to the converter
2.2µF
2.2µF
V
CM
1.25V
LTC2205/
LTC2204
22054 F07
PGA
REFERENCE
BANDGAP
REFERENCE
INTERNAL
2.5V
23
ADC
22054fb
22054 F06