LTC2335-16

22

233516f

For more information www.linear.com/LTC2335-16

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 16-

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 LTC2335-16 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-referred noise density, allowing it to approach the

full ADC data sheet SNR and THD specifications, when

used with a lowpass filter, as shown in the FFT plots in

Figures 6b to 6e. It may be used without a filter at a loss of

0.2dB SNR due to wideband noise. The LT1469 achieves

the full ADC specifications for DC precision, THD, and

linearity, at a cost of 0.5dB 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

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

94.5

−120

94.5

120

½ LT1469

0

0

FULLY DIFFERENTIAL

94.0

−124

94.0

120

½ LT1358

100

270

TRUE BIPOLAR

94.5

−107

94.3

108

½ LT1358

0

0

TRUE BIPOLAR

94.3

−108

94.2

110

½ LT1469

0

0

TRUE BIPOLAR

94.0

−109

94.0

110

½ LT1355

0

0

TRUE BIPOLAR

94.1

−103

93.6

104

Figure 5. True Bipolar Signal Chain with Input Filtering

–

+

LTC2335-16

233516 F05

ONLY CHANNEL 0 SHOWN FOR CLARITY

0V

TRUE BIPOLAR

INPUT SIGNAL

BUFFER

AMPLIFIER

LOWPASS

SIGNAL FILTER

10nF

160

BW = 100kHz