10

LTC1402

DRIVING THE ANALOG INPUT

The differential analog inputs of the LTC1402 are easy to

drive. The inputs may be driven differentially or as a single-

signal that is common to both inputs 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 LTC1402 inputs can be

driven directly. As source impedance increases, so will

acquisition time (see Figure 1). For minimum acquisition

time with high source impedance, a buffer amplifier must

be used. The only 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 50ns for full throughput rate).

Figure 1. Acquisition Time vs Source Resistance

in Bipolar and Unipolar Modes

SOURCE RESISTANCE (

Ω)

300

900

1100

1000

1300

1500

200

100

800

500

700

600

400

1200

1400

10

1k

10k

5V

100k

1402 F01

0

100

±5V

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 LTC1402 will depend on the

application. Generally, applications fall into two catego-

ries: 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 LTC1402. More detailed information is avail-

able in the Linear Technology Databooks and on the

LinearView

LT

down Pin (Amplifier Draws 200

µA While in Shutdown).

±5V to ±15V supplies. Distortion is –80dB to 1MHz

able with shutdown as LT1207. Output swings to within

LT1223: 100MHz Video Current Feedback Amplifier. 6mA

supply current.

±5V to ±15V supplies. Low distortion at

frequencies above 400kHz. Low noise. Good for AC appli-

cations.

LT1227: 140MHz Video Current Feedback Amplifier. 10mA

supply current; has shutdown pin (draws 120

µA while in

shutdown).

±5V to ±15V supplies. Lowest distortion

(– 92dB) at frequencies above 400kHz. Low noise. Best for

AC applications.

LT1229/LT1230: Dual and Quad 100MHz Current Feed-

back Amplifiers.

±2V to ±15V supplies. Low noise. Good

AC specifications, 6mA supply current each amplifier.

LT1360: 50MHz Voltage Feedback Amplifier. 3.8mA sup-

ply current.

±5V to ±15V supplies. Good AC and DC

specifications. 70ns settling to 0.5LSB.

LT1363: 70MHz, 1000V/

µs Op Amps. 6.3mA supply cur-

rent. Good AC and DC specifications. 60ns settling to

0.5LSB.

LT1364/LT1365: Dual and Quad 70MHz, 1000V/

µs Op

Amps. 6.3mA supply current per amplifier. 60ns settling

to 0.5LSB.

LinearView is a trademark of Linear Technology Corporation.

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