LTC2245 Linear Technology, LTC2245 Datasheet - Page 12

• Manufacturer Part Number: LTC2245
• Description: 10Msps Low Power 3V ADC
• Manufacturer: Linear Technology

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LTC2245
APPLICATIO S I FOR ATIO
Figure 4 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 band-
width of most op amps will limit the SFDR at high input
frequencies.
Figure 5 shows a single-ended input circuit. The imped-
ance seen by the analog inputs should be matched. This
circuit is not recommended if low distortion is required.
The 25Ω resistors and 12pF capacitor on the analog inputs
serve two purposes: isolating the drive circuitry from the
sample-and-hold charging glitches and limiting the
wideband noise at the converter input.
12
ANALOG
ANALOG
INPUT
INPUT
Figure 4. Differential Drive with an Amplifier
0.1µF
DIFFERENTIAL
CM
–
+
HIGH SPEED
Figure 5. Single-Ended Drive
AMPLIFIER
U
–
+
1k
U
25Ω
25Ω
1k
25Ω
25Ω
0.1µF
12pF
2.2µF
12pF
2.2µF
W
V
A
A
CM
IN
IN
V
A
A
+
–
CM
IN
IN
+
–
LTC2245
LTC2245
U
2245 F04
2245 F05
Reference Operation
Figure 6 shows the LTC2245 reference circuitry consisting
of a 1.5V bandgap reference, a difference amplifier and
switching and control circuit. The internal voltage refer-
ence can be configured for two pin selectable input ranges
of 2V (±1V differential) or 1V (±0.5V differential). Tying the
SENSE pin to V
pin to V
The 1.5V bandgap reference serves two functions: its
output provides a DC bias point for setting the common
mode voltage of any external input circuitry; additionally,
the reference is used with a difference amplifier to gener-
ate the differential reference levels needed by the internal
ADC circuitry. An external bypass capacitor is required for
the 1.5V reference output, V
frequency low impedance path to ground for internal and
external circuitry.
TIE TO V
TIE TO V
RANGE = 2 • V
CM
0.5V < V
DD
FOR 2V RANGE;
FOR 1V RANGE;
CM
2.2µF
1µF
1µF
1.5V
SENSE
SENSE
selects the 1V range.
Figure 6. Equivalent Reference Circuit
< 1V
FOR
DD
selects the 2V range; tying the SENSE
2.2µF
SENSE
0.1µF
REFH
REFL
V
CM
LTC2245
CONTROL
DETECT
4Ω
RANGE
AND
CM
. This provides a high
1.5V BANDGAP
DIFF AMP
REFERENCE
INTERNAL ADC
HIGH REFERENCE
INTERNAL ADC
LOW REFERENCE
1V
BUFFER
0.5V
2245 F06
2245fa