LTC1407ACMSE datasheet(11/24 Pages) LINER

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LTC1407/LTC1407A

1407f

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APPLICATIO S I FOR ATIO

DRIVING THE ANALOG INPUT

The differential analog inputs of the LTC1407/LTC1407A

are easy to drive. The inputs may be driven differentially or

as a single-ended input (i.e., the CH0– input is grounded).

All four analog inputs of both differential analog input

pairs, CH0+ with CH0– and CH1+ with CH1–, are sampled

at the same instant. Any unwanted signal that is common

to both inputs of each input pair will be reduced by the

common mode rejection of the sample-and-hold circuit.

The inputs draw only one small current spike while charg-

ing the sample-and-hold capacitors at the end of conver-

sion. During conversion, the analog inputs draw only a

small leakage current. If the source impedance of the

driving circuit is low, then the LTC1407/LTC1407A inputs

can be driven directly. As source impedance increases, so

will acquisition time. For minimum acquisition time with

high source impedance, a buffer amplifier must be used.

The main requirement is that the amplifier driving the

analog input(s) must settle after the small current spike

before the next conversion starts (settling time must be

39ns for full throughput rate). Also keep in mind, while

choosing an input amplifier, the amount of noise and

harmonic distortion added by the amplifier.

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

must be greater than 40MHz to ensure adequate small-

signal settling for full throughput rate. If slower op amps

are used, more time for settling can be provided by

increasing the time between conversions. The best choice

for an op amp to drive the LTC1407/LTC1407A depends

on the application. Generally, applications fall into two

categories: AC applications where dynamic specifications

are most critical and time domain applications where DC

accuracy and settling time are most critical. The following

list is a summary of the op amps that are suitable for

driving the LTC1407/LTC1407A. (More detailed informa-

tion is available in the Linear Technology Databooks and

on the LinearView

TM CD-ROM.)

LTC1566-1: Low Noise 2.3MHz Continuous Time Low-

pass Filter.

®1630: Dual 30MHz Rail-to-Rail Voltage FB Amplifier.

2.7V to

±15V supplies. Very high AVOL, 500µV offset and

520ns settling to 0.5LSB for a 4V swing. THD and noise

are – 93dB to 40kHz and below 1LSB to 320kHz (AV = 1,

2VP-P into 1kΩ, VS = 5V), making the part excellent for AC

applications (to 1/3 Nyquist) where rail-to-rail perfor-

mance is desired. Quad version is available as LT1631.

LT1632: Dual 45MHz Rail-to-Rail Voltage FB Amplifier.

2.7V to

±15V supplies. Very high AVOL, 1.5mV offset and

400ns settling to 0.5LSB for a 4V swing. It is suitable for

applications with a single 5V supply. THD and noise are

– 93dB to 40kHz and below 1LSB to 800kHz (AV = 1,

2VP-P into 1kΩ, VS = 5V), making the part excellent for AC

applications where rail-to-rail performance is desired.

Quad version is available as LT1633.

LT1801: 80MHz GBWP, –75dBc at 500kHz, 2mA/ampli-

fier, 8.5nV/

√Hz.

LT1806/LT1807: 325MHz GBWP, –80dBc distortion at

5MHz, unity gain stable, rail-to-rail in and out,

10mA/amplifier, 3.5nV/

√Hz.

LT1810: 180MHz GBWP, –90dBc distortion at 5MHz,

unity gain stable, rail-to-rail in and out, 15mA/amplifier,

16nV/

√Hz.

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