LTC2290UP View Datasheet(PDF) - Linear Technology
Part Name
Description
MFG CO.
'LTC2290UP' PDF : 24 Pages View PDF
LTC2290
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
VCM
2.2µF
AIN+
LTC2290
12pF
AIN–
2290 F04
Figure 4. Differential Drive with an Amplifier
Reference Operation
Figure 6 shows the LTC2290 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 VDD selects the 2V range; tying the SENSE
pin to VCM 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, VCM. 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
VCM
2.2µF
AIN+ LTC2290
25Ω
0.1µF
12pF
AIN–
2290 F05
LTC2290
1.5V
VCM
4Ω
1.5V BANDGAP
REFERENCE
2.2µF
1V 0.5V
TIE TO VDD FOR 2V RANGE;
TIE TO VCM FOR 1V RANGE;
RANGE = 2 • VSENSE FOR
0.5V < VSENSE < 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
2290 F06
Figure 6. Equivalent Reference Circuit
2290fa
14
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