LTC2335-18

22

233518f

For more information www.linear.com/LTC2335-18

contribution of the buffer. A simple one-pole lowpass RC

filter is sufficient for many applications.

This filter interacts with the buffer amplifier and slows

input settling. It is important that the inputs settle to

as insufficient settling can limit INL and THD performance.

High quality capacitors and resistors should be used in

the RC filters since these components can add distortion.

NPO/COG and silver mica type dielectric capacitors have

excellent linearity. Carbon surface mount resistors can

generate distortion from self-heating and from damage

that may occur during soldering. Metal film surface mount

resistors are much less susceptible to both problems.

Buffering Arbitrary and Fully Differential Analog Input

Signals

The wide common mode input range and high CMRR of

to swing with an arbitrary relationship to each other,

This unique feature of the LTC2335-18 enables it to accept

applicaTions inForMaTion

a wide variety of signal swings, simplifying signal chain

will not allow the channel’s sampling network to settle to

thesecases,itisrecommendedthattwounity-gainbuffers

be inserted between the signal source and the ADC input

pins, as shown in Figure 6a. Table 2 lists several amplifier

and lowpass filter combinations recommended for use in

this circuit.

The LT1358 combines fast settling, high linearity, and low

input-referrednoisedensity,allowingittoapproachthefull

ADC data sheet SNR and THD specifications when used

withalowpassfilter,asshownintheFFTplotsinFigures 6b

to 6e. It may be used without a filter at a loss of 0.3dB

SNR due to wideband noise. The LT1469 achieves the full

ADC specifications for DC precision, THD, and linearity,

at a cost of 0.8dB in SNR. Finally, the LT1355 provides a

good general-purpose combination of THD and SNR at a

lower power. Neither the LT1469 nor LT1355 can afford

the slowing effect of a lowpass filter if they are to be used

Table 2. Recommended Amplifier and Filter Combinations for the Buffer Circuits in Figures 6a and 9. AC Performance Measured

Using Circuit in Figure 6a, ±10.24V Range

AMPLIFIER

(Ω)

(pF)

INPUT SIGNAL DRIVE

SNR

(dB)

THD

(dB)

SINAD

(dB)

SFDR

(dB)

½ LT1358

100

270

FULLY DIFFERENTIAL

96.8

−120

96.8

120

½ LT1469

0

0

FULLY DIFFERENTIAL

96.0

−124

96.0

120

½ LT1358

100

270

TRUE BIPOLAR

96.8

−107

96.4

108

½ LT1358

0

0

TRUE BIPOLAR

96.5

−108

96.2

110

½ LT1469

0

0

TRUE BIPOLAR

96.0

−109

95.8

110

½ LT1355

0

0

TRUE BIPOLAR

96.2

−106

95.9

109

Figure 5. True Bipolar Signal Chain with Input Filtering

–

+

LTC2335-18

233518 F05

ONLY CHANNEL 0 SHOWN FOR CLARITY

0V

TRUE BIPOLAR

INPUT SIGNAL

BUFFER

AMPLIFIER

LOWPASS

SIGNAL FILTER

10nF

160

BW = 100kHz