LTC2225IUH#TRPBF【PDF 4/20 页】IC ADC 12BIT 10MSPS 3V 32-QFN

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LTC2225

2225fa

sample-and-hold charging glitches and limiting the

wideband noise at the converter input.

APPLICATIO S I FOR ATIO

The difference amplifier generates the high and low refer-

ence for the ADC. High speed switching circuits are

connected to these outputs and they must be externally

bypassed. Each output has two pins. The multiple output

pins are needed to reduce package inductance. Bypass

capacitors must be connected as shown in Figure 6.

Other voltage ranges in-between the pin selectable ranges

can be programmed with two external resistors as shown

in Figure 7. An external reference can be used by applying

its output directly or through a resistor divider to SENSE.

Figure 5. Single-Ended Drive

Figure 4. Differential Drive with an Amplifier

12pF

2.2

VCM

LTC2225

2225 F04

–

ANALOG

INPUT

HIGH SPEED

DIFFERENTIAL

AMPLIFIER

AIN

–

0.1

ANALOG

INPUT

VCM

AIN

–

12pF

2225 F05

2.2

0.1

LTC2225

Reference Operation

Figure 6 shows the LTC2225 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.

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

INTERNAL ADC

HIGH REFERENCE

BUFFER

0.1

2225 F06

LTC2225

DIFF AMP

INTERNAL ADC

LOW REFERENCE

1.5V BANDGAP

REFERENCE

0.5V

RANGE

DETECT

AND

CONTROL

Figure 6. Equivalent Reference Circuit

Figure 7. 1.5V Range ADC

VCM

SENSE

1.5V

0.75V

2.2

12k

2225 F07

LTC2225

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