LTC2295 datasheet(14/24 Pages) LINER

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LTC2295

2295fa

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. Single-Ended Drive

Figure 4. Differential Drive with an Amplifier

Ω

12pF

2.2

µF

VCM

2295 F04

–

ANALOG

INPUT

HIGH SPEED

DIFFERENTIAL

AMPLIFIER

AIN

–

LTC2295

Ω

0.1

µF

ANALOG

INPUT

VCM

AIN

–

12pF

2295 F05

2.2

µF

Ω

0.1

µF

LTC2295

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 (

±1Vdifferential)or1V(±0.5Vdifferential).Tyingthe

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 6. Equivalent Reference Circuit

VCM

REFH

SENSE

TIE TO VDD FOR 2V RANGE;

TIE TO VCM FOR 1V RANGE;

RANGE = 2 • VSENSE FOR

0.5V < VSENSE < 1V

1.5V

REFL

2.2

µF

2.2

µF

INTERNAL ADC

HIGH REFERENCE

BUFFER

0.1

µF

2295 F06

Ω

DIFF AMP

µF

INTERNAL ADC

LOW REFERENCE

1.5V BANDGAP

REFERENCE

0.5V

RANGE

DETECT

AND

CONTROL

LTC2295

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 25

Ωresistorsand12pFcapacitorontheanaloginputs

serve two purposes: isolating the drive circuitry from the

sample-and-hold charging glitches and limiting the

wideband noise at the converter input.

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