LTC1609ACSW datasheet(9/24 Pages) LINER

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LTC1609

1609fa

APPLICATIO S I FOR ATIO

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 shift

Driving the Analog Inputs

The LTC1609 analog input ranges, along with the nominal

input impedances, are shown in Tables 1a and 1b. The

inputs are overvoltage protected to

±25V. The input im-

pedance can get as low as 10k

Ω, therefore, it should 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

LTC1609. More detailed information is available in the

Linear Technology data books and LinearView

TM CD-ROM.

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.

LT1468 - 90MHz, 22V/

µs 16-bit accurate amplifier

LT1469 - Dual LT1468

Offset and Gain Adjustments

The LTC1609 is specified to operate with three unipolar

and three bipolar input ranges. Pins R1IN, R2IN and R3IN

are connected as shown in Tables 1a and 1b for the

different input ranges. The tables also list the nominal

input impedance for each range. Table 1c shows the

output codes for the ideal input voltages of each of the six

input ranges.

The LTC1609 offset and full-scale errors have been trimmed

at the factory with the external resistors shown in Figures

3a and 3b. This allows for external adjustment of offset and

full scale in applications where absolute accuracy is im-

portant. The offset and gain adjustment circuits for the six

input ranges are also shown in Figures 3a and 3b. To

adjust the offset for a bipolar input range, apply an input

voltage equal to – 0.5LSB where 1LSB = (+ FS – – FS)/

65536 and change the offset resistor so the output code is

changing between 1111 1111 1111 1111 and 0000 0000

0000 0000. The gain is trimmed by applying an input

voltage of + FS – 1.5LSB and adjusting the gain trim resis-

tor until the output code is changing between 0111 1111

1111 1110 and 0111 1111 1111 1111. In both cases the

data is in two’s complement format (SB/BTC = LOW)

To adjust the offset for a unipolar input range, apply an

input voltage equal to + 0.5LSB where 1LSB = + FS/65536.

Then adjust the offset trim resistor until the output code

changes between 0000 0000 0000 0000 and 0000 0000

0000 0001. To adjust the gain, apply an input voltage equal

to + FS – 1.5LSB and vary the gain trimming resistor until

the output code is changing between 1111 1111 1111 1110

and 1111 1111 1111 1111. In the unipolar case, the data

is in straight binary format (SB/BTC = HIGH). Figures 4a

and 4b show the transfer characteristics of the LTC1609.

R1IN

1000pF

AIN1

200

Ω

R2IN

1000pF

AIN2

100

Ω

LTC1609

R3IN

1000pF

AIN3

1609 F02

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.

LinearView is a trademark of Linear Technology Corporation.

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