LTC1411IG 查看數據表（PDF） - Linear Technology

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LTC1411IG

Single Supply 14-Bit 2.5Msps ADC

Linear Technology 

LTC1411

APPLICATIO S I FOR ATIO

LT6203: Dual 100MHz, Low Noise, Low Power Op Amp.

Specified at 3V, 5V and ±5V supplies. 1.9nV/√Hz noise

voltage.

Programmable Input Range

The LTC1411 has two logic input pins (PGA0 and PGA1)

that are used to select one of four analog input ranges.

These input ranges are set by changing the reference

voltage that is applied to the internal DAC of the ADC

(REFCOM2). For the “0dB” setting the internal DAC sees

the full reference voltage of 4V. The analog input range is

0.7V to 4.3V with AIN– = 2.5V. This corresponds to an input

span of ±1.8V with respect to the voltage applied to AIN–

. For the “– 3dB” setting the internal reference is reduced

to 0.707 • 4V = 2.9V. Likewise the input span is reduced

to ±1.28V. The following table lists the input span with

respect to AIN– for the different PGA0 and PGA1 settings.

Table 1. Input Spans for LTC1411

PGA0

PGA1

LEVEL

INPUT

SPAN

REFCOM2

VOLTAGE

5V

5V

0dB

±1.8V

4V

5V

0V

– 3dB

± 1.28V

2.9V

0V

5V

– 6dB

±0.9V

2V

0V

0V

– 9dB

± 0.64V

1.45V

When changing from one input span to another, more time

is needed for the REFCOM2 pin to reach the correct level

because the bypass capacitor on the pin needs to be charged

or discharged. Figure 6 shows the recommended capaci-

tors at the REFCOM1 and REFCOM2 pins (10µF each).

When – 6dB or – 9dB is selected, the voltage at REFCOM1

(see Figure 2) must first settle before REFCOM2 reaches

the correct level. The typical delay is about 700ms.

When the REFCOM2 level is changed from 2.9V to 4V

(changing PGA setting from – 3dB to 0dB), the typical delay

is 0.6ms. However, if the voltage at REFCOM2 is changed

from 4V to 2.9V (changing PGA setting from 0dB to – 3dB)

only a 60µA sink current is present to discharge the 10µF

bypass capacitor. In this case, the delay will be 11ms.

Internal Reference

The LTC1411 has an on-chip, temperature compensated,

curvature corrected, bandgap reference that is factory

trimmed to 2.500V. If this REFOUT pin is used to drive the

AIN– pin, a 22µF tantalum bypass capacitor is required and

this REFOUT voltage sets the bipolar zero for the ADC.

The REFIN pin is connected to the reference buffer through

a 2k resistor and two PGA switches. The REFIN pin can be

connected to REFOUT directly or to an external reference.

Figure 6 shows the reference and buffer structure for the

LTC1411. The input to the reference buffer is either REFIN

or 1/2 of REFIN depending on the PGA selection. The

REFCOM1 pin bypassed with a 10µF tantalum capacitor

helps reduce the noise going into the buffer. The reference

buffer has a gain of 1.62 or 1.15 (depends on PGA

selection). It is compensated at the REFCOM2 pin with a

10µF tantalum capacitor. The input span of the ADC is set

by the output voltage of this REFCOM2 voltage. For a 2.5V

input at the REFIN pin, the REFCOM2 will have 4V output

for PGA1 = PGA0 = 5V and the ADC will have a span of 3.6V.

REFOUT

22µF**

2.5V

BANDGAP

REFERENCE

REFIN*

5k

5k

2k

REFCOM1

10µF

REFCOM2

X1.62

10µF

1411 F06

*THIS PIN CAN BE TIED TO REFOUT OR AN EXTERNAL SOURCE

**A 22µF CAPACITOR IS NEEDED IF REFOUT IS USED TO DRIVE AIN–

Figure 6. Reference Structure for the LTC1411

for PGA1 = PGA0 = 5V

1411f

11

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