LTC2249CUH datasheet(12/20 Pages) LINER

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LTC2249

2249f

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.

APPLICATIO S I FOR ATIO

Reference Operation

Figure 9 shows the LTC2249 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.

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 9.

Other voltage ranges in-between the pin selectable ranges

can be programmed with two external resistors as shown

in Figure 10. An external reference can be used by applying

its output directly or through a resistor divider to SENSE.

It is not recommended to drive the SENSE pin with a logic

device. The SENSE pin should be tied to the appropriate

level as close to the converter as possible. If the SENSE pin

is driven externally, it should be bypassed to ground as

close to the device as possible with a 1µF ceramic capacitor.

Figure 5. Single-Ended Drive

25Ω

0.1µF

ANALOG

INPUT

VCM

AIN

–

10k

12pF

2249 F05

2.2µF

10k

25Ω

0.1µF

LTC2249

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.

For input frequencies above 70MHz, the input circuits of

Figure 6, 7 and 8 are recommended. The balun trans-

former gives better high frequency response than a flux

coupled center tapped transformer. The coupling capaci-

tors allow the analog inputs to be DC biased at 1.5V. In

Figure 8, the series inductors are impedance matching

elements that maximize the ADC bandwidth.

Figure 6. Recommended Front End Circuit for

Input Frequencies Between 70MHz and 170MHz

25Ω

12Ω

0.1µF

AIN

–

8pF

2.2µF

VCM

LTC2249

ANALOG

INPUT

0.1µF

T1 = MA/COM, ETC 1-1-13

RESISTORS, CAPACITORS

ARE 0402 PACKAGE SIZE

2249 F06

Figure 8. Recommended Front End Circuit for

Input Frequencies Above 300MHz

25Ω

0.1µF

AIN

–

2.2µF

VCM

LTC2249

ANALOG

INPUT

0.1µF

T1 = MA/COM, ETC 1-1-13

RESISTORS, CAPACITORS

ARE 0402 PACKAGE SIZE

2249 F07

25Ω

0.1µF

AIN

–

2.2µF

VCM

LTC2249

ANALOG

INPUT

0.1µF

T1 = MA/COM, ETC 1-1-13

RESISTORS, CAPACITORS, INDUCTORS

ARE 0402 PACKAGE SIZE

2249 F08

6.8nH

Figure 7. Recommended Front End Circuit for

Input Frequencies Between 170MHz and 300MHz

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