LTC2295_15 Datasheet, PDF (14/24 Pages) Linear Technology – Dual 14-Bit, 10Msps Low Power 3V ADC

LTC2295
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.
ANALOG
INPUT
HIGH SPEED
DIFFERENTIAL
AMPLIFIER 25Ω
++
CM
––
25Ω
V_CM
2.2µF
A_IN+
LTC2295
12pF
A_IN–
2295 F04
Figure 4. Differential Drive with an Amplifier
Reference Operation
Figure 6 shows the LTC2295 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_DD selects the 2V range; tying the SENSE
pin to V_CM selects the 1V range.
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_CM. This provides a high
frequency low impedance path to ground for internal and
external circuitry.
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.
ANALOG
INPUT
0.1µF
1k 1k
25Ω
V_CM
2.2µF
A_{IN+ LTC2295}
25Ω
0.1µF
12pF
A_IN–
2295 F05
LTC2295
1.5V
V_CM
4Ω
1.5V BANDGAP
REFERENCE
2.2µF
1V 0.5V
TIE TO V_DD FOR 2V RANGE;
TIE TO V_CM FOR 1V RANGE;
RANGE = 2 • V_SENSE FOR
0.5V < V_SENSE < 1V
SENSE
1µF
REFH
RANGE
DETECT
AND
CONTROL
BUFFER
INTERNAL ADC
HIGH REFERENCE
Figure 5. Single-Ended Drive
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.
2.2µF
1µF
0.1µF
REFL
DIFF AMP
INTERNAL ADC
LOW REFERENCE
2295 F06
Figure 6. Equivalent Reference Circuit
2295fa
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