LTC2246LX(RevB) View Datasheet(PDF) - Linear Technology

Part Name

Description

Manufacturer

LTC2246LX
(Rev.:RevB)

14-Bit, 25Msps 125°C ADC In LQFP

Linear Technology 

LTC2246H

APPLICATIONS INFORMATION

Reference Operation

Figure 6 shows the LTC2246H reference circuitry consist-

ing of a 1.5V bandgap reference, a difference amplifier and

switching and control circuit. The internal voltage reference

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

reference for the ADC. High speed switching circuits are

connected to these outputs and they must be externally

bypassed.

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 apply-

ing 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.

Input Range

The input range can be set based on the application.

The 2V input range will provide the best signal-to-noise

performance while maintaining excellent SFDR. The 1V

input range will have better SFDR performance, but the

SNR will degrade by 5.8dB.

LTC2246H

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

2.2μF

1μF

0.1μF

REFL

DIFF AMP

INTERNAL ADC

LOW REFERENCE

Figure 6. Equivalent Reference Circuit

2246H F06

1.5V

12k

0.75V

12k

VCM

2.2μF

SENSE LTC2246H

1μF

2246H F07

Figure 7. 1.5V Range ADC

4.7μF

CLEAN

SUPPLY

FERRITE

BEAD

0.1μF

100Ω

CLK LTC2246H

2246H F08

IF LVDS USE FIN1002 OR FIN1018.

FOR PECL, USE AZ1000ELT21 OR SIMILAR

Figure 8. CLK Drive Using an LVDS or PECL to CMOS Converter

2246hfb

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