LTC2345-18

23

234518f

For more information www.linear.com/LTC2345-18

enough to allow the analog inputs to completely settle to

as insufficient settling can limit INL and THD performance.

Also note that the minimum acquisition time varies with

channels.

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,

unique feature of the LTC2345-18 enables it to accept a

wide variety of signal swings, simplifying signal chain

applicaTions inForMaTion

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 LT6237 combines fast settling, high

linearity, and low offset with 1.1nV/√Hz input-referred

noise density, enabling it to achieve the full ADC data

sheet SNR and THD specifications, as shown in the FFT

plots in Figures 6b to 6e. In applications where slightly

degraded SNR performance is acceptable, it is possible

to drive the LTC2345-18 using the lower-power LT6234.

The LT6234 combines fast settling, good linearity, and

low offset with 1.9nV/√Hz input-referred noise density,

enabling it to drive the LTC2345-18 with only 0.4dB SNR

loss compared with the LT6237 when a 40.2Ω, 1nF filter

is employed. As shown in Table 2, the LT6237 may be

used without a lowpass filter at a loss of ≤1dB SNR due

to increased wideband noise.

Figure 5. Unipolar Signal Chain with Input Filtering

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

Using Circuit in Figure 6a, ±4.096V Range for Fully Differential Input Drive, ±2.048V Range for Bipolar Input Drive

AMPLIFIER

(Ω)

(nF)

INPUT SIGNAL DRIVE

SNR

(dB)

THD

(dB)

SINAD

(dB)

SFDR

(dB)

½ LT6237

40.2

1

FULLY DIFFERENTIAL

92.0

−114

92.0

115

½ LT6234

40.2

1

FULLY DIFFERENTIAL

91.7

−114

91.7

115

½ LT6237

40.2

1

BIPOLAR

86.1

−108

86.1

109

½ LT6234

40.2

1

BIPOLAR

85.7

−108

85.7

109

½ LT6237

0

0

BIPOLAR

85.6

−108

85.6

109

½ LT6234

0

0

BIPOLAR

81.5

−107

81.5

108

–

+

LTC2345-18

234518 F05

ONLY CHANNEL 0 SHOWN FOR CLARITY

5V

0V

UNIPOLAR

INPUT SIGNAL

BUFFER

AMPLIFIER

LOWPASS

SIGNAL FILTER

10nF

160

BW = 100kHz