LTC2274 Linear Technology, LTC2274 Datasheet - Page 20

• Manufacturer Part Number: LTC2274
• Description: 105Msps Serial Output ADC
• 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 LTC2274 reference circuitry consisting
of a 2.5V bandgap reference, a programmable gain ampli-
fi er and control circuit. The LTC2274 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
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
= 0). A 1.25V output V
bias for input drive circuitry. An external bypass capacitor is
required for the V
low impedance path to ground for internal and external
circuitry. This is also the compensation capacitor for the
LTC2274
20
ANALOG
Figure 5. DC Coupled Input with Differential Amplifi er
INPUT
AMPLIFIER = LTC6600-20,
LTC1993, ETC.
DIFFERENTIAL
+
CM
–
HIGH SPEED
CM
AMPLIFIER
output. This provides a high frequency
+
–
CM
is provided for a common mode
25Ω
25Ω
DD
2.2μF
12pF
12pF
. To use an external
V
A
A
CM
IN
IN
+
–
LTC2274
P-P
2274 F05
(PGA
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.
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.
PGA Pin
The PGA pin selects between two gain settings for
the ADC front-end. PGA = 0 selects an input range of
2.25V
2.25V input range has the best SNR; however, the distor-
tion will be higher for input frequencies above 100MHz.
For applications with high input frequencies, the low
input range will have improved distortion; however, the
SNR will be 2.4dB worse. See the Typical Performance
Characteristics section of this datasheet.
TIE TO V
EXTERNAL 1.25V
EXTERNAL 2.5V
INTERNAL 2.5V
OR INPUT FOR
OR INPUT FOR
REFERENCE
REFERENCE
REFERENCE
DD
P-P
TO USE
; PGA = 1 selects an input range of 1.5V
SENSE
2.2μF
V
CM
Figure 6. Reference Circuit
LTC2274
BUFFER
DD
AND GAIN
CONTROL
SELECT
RANGE
as close to the converter
1.25V
1x OR 2x
REFERENCE
BANDGAP
REFERENCE
INTERNAL
2.5V
P-P
ADC
. The
2274 F06
2274f