10

LTC1405

Differential Operation

The THD and SFDR performance of the LTC1405 can be

improved by using a center tap RF transformer to drive the

inputs differentially. Though the signal can no longer be

DC coupled, the improvement in dynamic performance

makes this an attractive solution for some applications.

Typical connections for single and dual supply systems

are shown in Figures 8a and 8b. Good choices for trans-

formers are the Mini Circuits T1-1T (1:1 turns ratio) and

T4-6T (1:4 turns ratio). For best results the transformer

should be located close to the LTC1405 on the printed

circuit board.

must be greater than 50MHz to ensure adequate small-

signal settling for full throughput rate. If slower op amps

are used, more settling time can be provided by increasing

the time between conversions.

The best choice for an op amp to drive the LTC1405 will

depend on the application. Generally applications fall into

two categories: AC applications where dynamic specifica-

tions are most critical and time domain applications where

DC accuracy and settling time are most critical.

Input Filtering

The noise and the distortion of the input amplifier and

other circuitry must be considered since they will add to

the LTC1405 noise and distortion. The small-signal band-

width of the sample-and-hold circuit is 100MHz. Any noise

or distortion products that are present at the analog inputs

will be summed over this entire bandwidth. Noisy input

circuitry should be filtered prior to the analog inputs to

minimize noise. A simple 1-pole RC filter is sufficient for

many applications.

For example, Figure 9 shows a 1000pF capacitor from

bandwidth to 5.3MHz. The 1000pF capacitor also acts as

a charge reservoir for the input sample-and-hold and

current glitch. In undersampling applications, an input

capacitor this large may prohibitively limit the input band-

width. If this is the case, use as large an input capacitance

as possible. High quality capacitors and resistors should

be used since these components can add distortion. NPO

and silver mica type dielectric capacitors have excellent

linearity. Carbon surface mount resistors can generate

distortion from self-heating and from damage that may

occur during soldering. Metal film surface mount resis-

tors are much less susceptible to both problems.

APPLICATIO S I FOR ATIO

1405 F08a

1000pF

15

Ω

15

Ω

MINI CIRCUITS

T1-1T

1

µF

LTC1405

5V

1405 F08b

MINI CIRCUITS

T1-1T

1

µF

LTC1405

5V

–5V

1000pF

15

Ω

15

Ω

Figure 8a. Single Supply Transformer Coupled Input

Figure 8b. Dual Supply Transformer Coupled Input

LTC1405

1405 F09

1000pF

30

Ω

Choosing an Input Amplifier

Choosing an input amplifier is easy if a few requirements

are taken into consideration. First, to limit the magnitude

of the voltage spike seen by the amplifier from charging

the sampling capacitor, choose an amplifier that has a low

output impedance (<100

Ω) at the closed-loop bandwidth

frequency. For example, if an amplifier is used in a gain of

1 and has a unity-gain bandwidth of 50MHz, then the

output impedance at 50MHz must be less than 100

Ω. The

second requirement is that the closed-loop bandwidth

Figure 9. RC Input Filter