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LTC1744

1744f

APPLICATIO S I FOR ATIO

Input Drive Circuits

Figure 3 shows the LTC1744 being driven by an RF

transformer with a center tapped secondary. The second-

ary center tap is DC biased with VCM, setting the ADC input

signal at its optimum DC level. Figure 3 shows a 1:1 turns

ratio transformer. Other turns ratios can be used if the

source impedence seen by the ADC does not exceed 100

Ω

for each ADC input. A disadvantage of using a transformer

is the loss of low frequency response. Most small RF

transformers have poor performance at frequencies be-

low 1MHz.

frequencies higher than 40MHz, the capacitors may need

to be decreased to prevent excessive signal loss.

Reference Operation

Figure 5 shows the LTC1744 reference circuitry consisting

of a 2.5V 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 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 2.5V 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.

1:1

Ω

0.1

µF

ANALOG

INPUT

VCM

AIN

–

100

Ω

100

Ω

18pF

1744 F03

4.7

µF

Ω

LTC1744

Ω

SINGLE-ENDED

INPUT

2.5V

±1/2 RANGE

VCM

AIN

–

18pF

1744 F04a

4.7

µF

Ω

100

Ω

500

Ω

500

Ω

LTC1744

–

1/2 LT1810

–

1/2 LT1810

Figure 3. Single-Ended to Differential

Conversion Using a Transformer

Figure 4a. Differential Drive with Op Amps

Figure 4b. Using the LT6600 as a Differential Driver

GAIN = 402

Ω/RIN

MAXIMUM GAIN = 4

VCM

AIN

–

18pF

1744 F04b

µF

49.9

Ω

49.9

Ω

RIN

402

Ω

RIN

402

Ω

0.1

µF

0.01

µF

LTC1744

–

LT6600-20

VOCM

VMID

VIN

Figure 4a demonstrates the use of operational amplifiers

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 bandwidth of most op amps will limit the

SFDR at high input frequencies.

Figure 4b shows the LT6600, a low noise differential

amplifier and lowpass filter, used as an input driver. The

LT6600 provides two functions: it serves as a 4th order

lowpass filter and as a single-ended to differential con-

verter. Additionally it can be programmed with one exter-

nal resistor to provide a gain from 1 to 4. Three versions

of this device are available having lowpass filter band-

widths of 2.5MHz, 10MHz or 20MHz.

The 37

Ω resistors and 18pF capacitors 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

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