10

LTC1410

capacitors at the end of conversion. During conversion

the analog inputs draw only a small leakage current. If the

source impedance of the driving circuit is low then the

LTC1410 inputs can be driven directly. As source imped-

ance increases so will acquisition time (see Figure 6). For

minimum acquisition time with high source impedance,

a buffer amplifier should be used. The only requirement

is that the amplifier driving the analog input(s) must

settle after the small current spike before the next conver-

sion starts (settling time must be 100ns for full through-

put rate).

SOURCE RESISTANCE (

Ω)

10

1

1410 F06

0.1

0.01

100

1k

10k

100k

10

Figure 6. Acquisition Time vs Source Resistance

Choosing an input amplifier is easy if a few requirements

are taken into consideration. First, 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 closed-loop bandwidth of

50MHz, then the output impedance at 50MHz must be

less than 100

Ω. The second requirement is that the

closed-loop bandwidth must be greater than 20MHz 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.

Suitable devices capable of driving the ADC’s inputs

include the LT

LT1227 op amps.

The noise and the distortion of the input amplifier must

also be considered since they will add to the LTC1410

noise and distortion. The small-signal bandwidth of the

sample-and-hold circuit is 20MHz. Any noise that is 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 usually sufficient. For example, Figure 7 shows a

resistor will limit the input bandwidth to 1.6MHz. Simple

RC filters work well for AC applications, but they will limit

the transient response. For full speed operation, amplifiers

with fast settling and low noise should be chosen.

1

2

3

4

5

0.1

µF

10

µF

100

Ω

ANALOG

INPUT

1000pF

1410 F07

REFCOMP

AGND

LTC1410

Figure 7. RC Input Filter

Internal Reference

The LTC1410 has an on-chip, temperature compensated,

curvature corrected, bandgap reference which is factory

trimmed to 2.500V. It is connected internally to a reference

2k resistor is in series with the output so that it can be

1

2

3

LTC1410

4.06V

0.1

µF

10

µF

R2

40k

R1

2k

ANALOG

INPUT

2.500V

1410 F08a

REFCOMP

AGND

4

5

R3

64k

+

–

BANDGAP

REFERENCE

Figure 8a. LTC1410 Reference Circuit