LTC1741CFW datasheet(13/20 Pages) LINER

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LTC1741

1741f

Reference Operation

Figure 5 shows the LTC1741 reference circuitry consisting

of a 2.35V 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 3.2V(±1.6V differential). Tying

the SENSE pin to ground selects the 2V range; tying the

SENSE pin to VDD selects the 3.2V range.

The 2.35V 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 2.35V

reference output, VCM. This provides a high frequency low

impedance path to ground for internal and external cir-

cuitry. This is also the compensation capacitor for the

reference. It will not be stable without this capacitor.

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: REFHA and REFHB

for the high reference and REFLA and REFLB for the low

reference. The doubled output pins are needed to reduce

package inductance. Bypass capacitors must be con-

nected as shown in Figure 5.

Other voltage ranges in between the pin selectable ranges

can be programmed with two external resistors as shown

in Figure 6a. 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 since the logic threshold is close to ground and

VDD. The SENSE pin should be tied high or low 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 1

µF ceramic capacitor.

Input Range

The input range can be set based on the application. For

oversampled signal processing in which the input fre-

quency is low (<10MHz), the largest input range will

provide the best signal-to-noise performance while main-

taining excellent SFDR. For high input frequencies

(> 40MHz), the 2V range will have the best SFDR perfor-

mance for the 2nd and 3rd harmonics, but the SNR will

degrade by 1.5dB. See the Typical Performance Charac-

teristics section.

APPLICATIO S I FOR ATIO

Ω

SINGLE-ENDED

INPUT

2.35V

±1/2

RANGE

VCM

AIN

–

12pF

1741 F04

4.7

µF

Ω

100

Ω

500

Ω

500

Ω

LTC1741

–

1/2 LT1810

–

1/2 LT1810

Figure 4. Differential Drive with Op Amps

VCM

REFHA

REFLB

SENSE

TIE TO VDD FOR 3.2V RANGE;

TIE TO GND FOR 2V RANGE;

RANGE = 2 • VSENSE FOR

1V < VSENSE < 1.6V

2.35V

REFLA

REFHB

4.7

µF

4.7

µF

INTERNAL ADC

HIGH REFERENCE

BUFFER

0.1

µF

1741 F05

LTC1741

Ω

DIFF AMP

µF

0.1

µF

INTERNAL ADC

LOW REFERENCE

2.35V BANDGAP

REFERENCE

1.6V

RANGE

DETECT

AND

CONTROL

Figure 5. Equivalent Reference Circuit

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