LTC1605ACSW datasheet(8/16 Pages) LINER

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LTC1605

1605fc

APPLICATIONS INFORMATION

Driving the Analog Inputs

The nominal input range for the LTC1605 is

±10V or

(

±4 • VREF)andtheinputisovervoltageprotectedto±25V.

The input impedance is typically 20k

Ω,therefore,itshould

be driven with a low impedance source. Wideband noise

coupling into the input can be minimized by placing a

1000pF capacitor at the input as shown in Figure 2. An

NPO-type capacitor gives the lowest distortion. Place the

capacitor as close to the device input pin as possible. If an

amplifier is to be used to drive the input, care should be

taken to select an amplifier with adequate accuracy, linear-

ity and noise for the application. The following list is a

summary of the op amps that are suitable for driving the

LTC1605. More detailed information is available in the

Linear Technology data books and LinearView

TM CD-ROM.

Conversion Details

The LTC1605 uses a successive approximation algorithm

and an internal sample-and-hold circuit to convert an

analog signal to a 16-bit or two byte parallel output. The

ADC is complete with a precision reference and an internal

clock. The control logic provides easy interface to micro-

processors and DSPs. (Please refer to the Digital Interface

section for the data format.)

Conversion start is controlled by the CS and R/C inputs. At

the start of conversion the successive approximation

it cannot be restarted.

During the conversion, the internal 16-bit capacitive DAC

output is sequenced by the SAR from the most significant

bit (MSB) to the least significant bit (LSB). Referring to

Figure 1, VIN is connected through the resistor divider to

the sample-and-hold capacitor during the acquire phase

and the comparator offset is nulled by the autozero switches.

In this acquire phase, a minimum delay of 2

µs will provide

enough time for the sample-and-hold capacitor to acquire

the analog signal. During the convert phase, the autozero

switches open, putting the comparator into the compare

mode. The input switch switches CSAMPLE to ground,

injecting the analog input charge onto the summing junc-

tion. This input charge is successively compared with the

binary-weighted charges supplied by the capacitive DAC.

Bit decisions are made by the high speed comparator. At

the end of a conversion, the DAC output balances the VIN

input charge. The SAR contents (a 16-bit data word) that

represents the VIN are loaded into the 16-bit output latches.

Figure 1. LTC1605 Simplified Equivalent Circuit

VDAC

1605 • F01

–

CDAC

DAC

SAMPLE

HOLD

CSAMPLE

16-BIT

LATCH

COMPARATOR

SAMPLE

RIN2

RIN1

VIN

1605 • F02

1000pF

33.2k

VIN

CAP

AIN

200

Ω

Figure 2. Analog Input Filtering

LT1007 - Low noise precision amplifier. 2.7mA supply

current

±5V to ±15V supplies. Gain bandwidth product

8MHz. DC applications.

LT1097 - Low cost, low power precision amplifier. 300

µA

supply current.

±5V to ±15V supplies. Gain bandwidth

product 0.7MHz. DC applications.

LT1227 - 140MHz video current feedback amplifier. 10mA

supply current.

±5V to ±15V supplies. Low noise and low

distortion.

LT1360 - 37MHz voltage feedback amplifier. 3.8mA sup-

ply current.

±5V to ±15V supplies. Good AC/DC specs.

LT1363 - 50MHz voltage feedback amplifier. 6.3mA sup-

ply current. Good AC/DC specs.

LT1364/LT1365 - Dual and quad 50MHz voltage feedback

amplifiers. 6.3mA supply current per amplifier. Good AC/

DC specs.

LinearView is a trademark of Linear Technology Corporation

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