LTM9012 datasheet(14/28 Pages) LINER

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LTM9012

9012f

applicaTions inForMaTion

CONVERTER OPERATION

The LTM9012 is a low power, 4-channel, 14-bit, 125Msps

A/D converter that is powered by a 1.8V ADC supply and

3.3V driver supplies. Each input includes a fixed gain,

differential amplifier. The analog inputs can be driven dif-

ferentially or single-ended. The encode input can be driven

differentiallyforoptimaljitterperformance,orsingle-ended

forlowerpowerconsumption.Thedigitaloutputsareserial

LVDS to minimize the number of data lines. Each channel

outputstwobitsatatime(2-lanemode).Atlowersampling

rates there is a one bit per channel option (1-lane mode).

Many additional features can be chosen by programming

the mode control registers through a serial SPI port.

Analog Inputs

The analog inputs for each channel of the LTM9012 con-

sist of a differential amplifier with fixed gain followed by

a lowpass filter. The 10x gain version has 49.9Ω series

resistance in each input.

The differential input can support single-ended operation

by connecting the inverting input to a fixed DC voltage or

ground. However, if ground is used, there will be a 6dB

loss of dynamic range. For maximum dynamic range, con-

nect the inverting inputs of the LTM9012 to a DC voltage

equal to the median of the voltage excursions of the non-

inverting input. An op amp provides an excellent means

of providing a low impedance voltage source capable of

sourcing and sinking small amounts of current. Note the

value of this DC voltage should fall between the limits of

allowable input common mode voltages. See Figure 2 for

an example.

LTM9012

(1 CHANNEL SHOWN)

9012 F02

–

¼LTC6254

VREF

SIGNAL

0.1µF

SET VREF EQUAL TO THE DC MEDIAN OF THE SIGNAL VOLTAGE

Figure 2. Single-Ended Operation

The gain of the LTM9012 may also be decreased from

the nominal value by adding resistance in series with

the signal inputs. The internal op amps are fed by 49.9Ω

series resistors and employ 511Ω feedback resistors. The

voltage gain of the stage is set by the ratio of the feedback

resistance to the total series resistance. Unity gain, for

example, can be realized by adding a 464Ω resistor in

series with each input.

Reference

The LTM9012 has an internal 1.25V voltage reference. For

a 2V input range using the internal reference with a unity

gaininternalamplifierconfiguration,connectSENSEtoVDD.

For a 1V input range using the internal reference, connect

SENSE to ground. For a 2V input range with an external

reference, apply a 1.25V reference voltage to SENSE.

The input range can be adjusted by applying a voltage to

SENSE that is between 0.625V and 1.30V. The input range

will then be 1.6 • VSENSE.

The reference is shared by all four ADC channels, so it is

not possible to independently adjust the input range of

individual channels.

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