LTC2204CUK View Datasheet(PDF) - Linear Technology
Part Name
Description
MFG CO.
'LTC2204CUK' PDF : 36 Pages View PDF
LTC2205/LTC2204
APPLICATIONS INFORMATION
Direct Coupled Circuits
Figure 5 demonstrates the use of a differential amplifier 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 amplifier will
degrade the ADC SFDR at high input frequencies. Additionally,
wideband op amps or differential amplifiers tend to have
high noise. As a result, the SNR will be degraded unless the
noise bandwidth is limited prior to the ADC input.
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 VDD 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.
ANALOG
INPUT
HIGH SPEED
DIFFERENTIAL
AMPLIFIER 25Ω
++
CM
––
25Ω
AMPLIFIER = LTC6600-20,
LTC1993, ETC.
VCM
2.2μF
AIN+
12pF
LTC2205/
LTC2204
AIN–
12pF
22054 F05
Figure 5. DC Coupled Input with Differential Amplifier
Reference Operation
Figure 6 shows the LTC2205/LTC2204 reference circuitry
consisting of a 2.5V bandgap reference, a programmable
gain amplifier and control circuit. The LTC2205/LTC2204
have 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 VDD. 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 2.25VP-P (PGA = 0). A 1.25V output,
VCM is provided for a common mode bias for input drive
circuitry. An external bypass capacitor is required for the
VCM 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 amplifier provides the
internal reference voltage for the ADC. This amplifier has
LTC2205/
LTC2204
TIE TO VDD TO USE
INTERNAL 2.5V
REFERENCE
OR INPUT FOR
EXTERNAL 2.5V
REFERENCE
OR INPUT FOR
EXTERNAL 1.25V
REFERENCE
SENSE
RANGE
SELECT
AND GAIN
CONTROL
VCM
2.2μF
BUFFER
1.25V
INTERNAL
ADC
REFERENCE
PGA
2.5V
BANDGAP
REFERENCE
22054 F06
Figure 6. Reference Circuit
3.3V
1μF
1.25V
2 LT1461-2.5 6
4
VCM
2.2μF
SENSE
2.2μF
LTC2205/
LTC2204
22054 F07
Figure 7. A 2.25V Range ADC with
an External 2.5V Reference
22054fc
23
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