LTC2348-18

22

234818fa

For more information www.linear.com/LTC2348-18

while simultaneously filtering driver wideband noise. A

buffer amplifier with low noise density should be selected

to minimize SNR degradation over this bandwidth. An

additional filter network may be placed between the buf-

fer output and ADC input to further minimize the noise

contribution of the buffer and reduce disturbances to the

buffer from ADC acquisition transients. A simple one-pole

lowpass RC filter is sufficient for many applications. It is

important that the RC time constant of this filter be small

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.

applicaTions inForMaTion

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 LTC2348-18 enables it to accept

a wide variety of signal swings, simplifying signal chain

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

In these cases, it is recommended that two unity-gain

buffers 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 LT1469 combines fast settling,

high linearity, and low offset with 5nV/√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

LTC2348-18

234818 F05

ONLY CHANNEL 0 SHOWN FOR CLARITY

0V

TRUE BIPOLAR

INPUT SIGNAL

BUFFER

AMPLIFIER

LOWPASS

SIGNAL FILTER

10nF

160

BW = 100kHz

Figure 5. True Bipolar 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, ±10.24V Range

AMPLIFIER

(Ω)

(pF)

INPUT SIGNAL DRIVE

SNR

(dB)

THD

(dB)

SINAD

(dB)

SFDR

(dB)

½ LT1469

49.9

1000

FULLY DIFFERENTIAL

96.7

−119

96.6

120

½ LT1355

100

270

FULLY DIFFERENTIAL

96.5

−119

96.4

120

½ LT1469

49.9

1000

TRUE BIPOLAR

96.7

−109

96.5

110

½ LT1355

100

270

TRUE BIPOLAR

96.5

−106

96.1

108

½ LT1469

0

0

TRUE BIPOLAR

95.7

−109

95.5

110

½ LT1355

0

0

TRUE BIPOLAR

95.9

−106

95.5

108

½ LT1358

100

270

TRUE BIPOLAR

96.7

−108

96.5

110