LTC1853_15 datasheet(12/24 Pages) LINER

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LTC1852/LTC1853

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The LTC1852/LTC1853 have a unique differential sample-

and-hold circuit that allows rail-to-rail inputs. The ADC will

always convert the difference of the “+” and “–” inputs

independent of the common mode voltage. The common

mode rejection holds up to high frequencies. The only

requirement is that both inputs can not exceed the AVDD

power supply voltage or ground. When a bipolar input

span is selected the “+” input can swing ± full scale rela-

tive to the “–” input but neither input can exceed AVDD or

go below ground.

Integral nonlinearity errors (INL) and differential nonlin-

earity errors (DNL) are independent of the common mode

voltage, however, the bipolar offset will vary. The change

in bipolar offset is typically less than 0.1% of the common

mode voltage.

Some AC applications may have their performance limited

by distortion. Most circuits exhibit higher distortion when

signals approach the supply or ground. THD will degrade

as the inputs approach either power supply rail. Distor-

tion can be reduced by reducing the signal amplitude

and keeping the common mode voltage at approximately

midsupply.

Driving the Analog Inputs

The inputs of the LTC1852/LTC1853 are easy to drive. Each

of the analog inputs can be used as a single-ended input

relative to the input common pin (CH0-COM, CH1-COM,

etc.) or in pairs (CH0 and CH1, CH2 and CH3, CH4 and

CH5, CH6 and CH7) for differential inputs. Regardless

of the MUX conﬁguration, the “+” and “–” inputs are

sampled at the same instant. Any unwanted signal that is

common mode to both inputs will be reduced by the com-

mon mode rejection of the sample-and-hold circuit. The

inputs draw only one small current spike while charging

the sample-and-hold capacitors at the end of conversion.

During conversion, the analog inputs draw only a small

leakage current. If the source impedance of the driving

circuit is low, then the LTC1852/LTC1853 inputs can be

driven directly. As source impedance increases, so will

acquisition time. For minimum acquisition time with high

source impedance, a buffer ampliﬁer should 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 less than

150ns for full throughput rate).

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 (<100Ω) 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 should be less than 100Ω.

The second requirement is that the closed-loop bandwidth

must be greater than 10MHz to ensure adequate small-

signal settling for full throughput rate. The following list

is a summary of the op amps that are suitable for driving

the LTC1852/LTC1853, more detailed information is avail-

able in the Linear Technology Databooks, the LinearView™

CD-ROM and on our web site at www.linear-tech.com.

LT®1360:

50MHz Voltage Feedback Ampliﬁer. ± 2.5V to

±15V supplies. 5mA supply current. Low distortion.

LT1363: 70MHz Voltage Feedback Ampliﬁer. ± 2.5V to ±15V

supplies. 7.5mA supply current. Low distortion.

LT1364/LT1365: Dual and Quad 70MHz Voltage Feedback

Ampliﬁers. ±2.5V to ±15V supplies. 7.5mA supply current

per ampliﬁer. Low distortion.

LT1468/LT1469: Single and Dual 90MHz Voltage Feedback

Ampliﬁer. ± 5V to ±15V supplies. 7mA supply current per

ampliﬁer. Lowest noise and low distortion.

LT1630/LT1631: Dual and Quad 30MHz Rail-to-Rail Volt-

age Feedback Ampliﬁers. Single 3V to ±15V supplies.

3.5mA supply current per ampliﬁer. Low noise and low

distortion.

LT1632/LT1633: Dual and Quad 45MHz Rail-to-Rail Voltage

Feedback Ampliﬁers. Single 3V to ±15V supplies. 4.3mA

supply current per ampliﬁer. Low distortion.

LT1806/LT1807: Single and Dual 325MHz Rail-to-Rail

Voltage Feedback Ampliﬁer. Single 3V to ± 5V supplies.

13mA supply current. Lowest distortion.

LinearView is a trademark of Linear Technology Corporation.

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