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LTC2245

2245fa

APPLICATIO S I FOR ATIO

WUUU

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

selects the 2V range; tying the SENSE

pin to V

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

. This provides a high

frequency low impedance path to ground for internal and

external circuitry.

Figure 5. Single-Ended Drive

Figure 4. Differential Drive with an Amplifier

25Ω

12pF

2.2µF

LTC2245

2245 F04

–

ANALOG

INPUT

HIGH SPEED

DIFFERENTIAL

AMPLIFIER

–

25Ω

0.1µF

ANALOG

INPUT

–

12pF

2245 F05

2.2µF

25Ω

0.1µF

LTC2245

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.

REFH

SENSE

TIE TO V

FOR 2V RANGE;

TIE TO V

FOR 1V RANGE;

RANGE = 2 • V

SENSE

FOR

0.5V < V

SENSE

< 1V

1.5V

REFL

2.2µF

INTERNAL ADC

HIGH REFERENCE

BUFFER

0.1µF

2245 F06

LTC2245

4Ω

DIFF AMP

1µF

INTERNAL ADC

LOW REFERENCE

1.5V BANDGAP

REFERENCE

0.5V

RANGE

DETECT

AND

CONTROL

Figure 6. Equivalent Reference Circuit

1 2 ... 7 8 9 10 11 12 13 14 15 16 17 18 19 20

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